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Allow emitting & parsing of bytes per dagjson codec spec

fix #165

These are somewhat overdue, and clarify what features are supported,
and also note some discrepancies in implementation versus the spec.

(As I'm taking this inventory of discrepancies, there's admittedly
rather more than I'd like... but step 1: document the current truth.
Prioritizing which things to hack on, in the field of infinite possible
prioritizations of things that need hacking on, can be a step 2.)

These had drifted a bit, and I just noticed it's due for some freshening up.

schema/gen/go: apply gofmt automatically

Now that we buffer the output, using go/format is trivial.

This makes the default behavior better, and means not having to use an
extra gofmt go:generate step everywhere.

And re-generate all code in this module.

This gets us to a point where go-codec-dagpb has zero vet warnings, for
example. schema/dmt still has a few warnings, but those are trickier to
fix, so will require another PR.

schema/gen/go: batch file writes via a bytes.Buffer

With this change, running 'go generate ./...' on the entire module while
running gopls on one of its files drops gopls's CPU spinning from ~25s
to well under a second. They should improve that anyway, but there's no
reason for the tens of thousands of tiny FS writes on our end either.

The time to run 'go generate ./...' itself is largely unaffected; it
goes from ~1.2s to ~1.1s, judging by a handful of runs.

If we know that a schema type can be represented in Go with a small
amount of bytes, using a pointer to store its "maybe" is rarely a good
idea. For example, an optional string only weighs twice as much as a
pointer, so a pointer adds overhead and will barely ever save any
memory.

Add a function to work out the byte size of a schema.TypeKind, relying
on reflection and the basicnode package. Debug prints are also present
if one wants to double-check the numbers. As of today, they are:

	sizeOf(small): 32 (4x pointer size)
	sizeOf(Bool): 1
	sizeOf(Int): 8
	sizeOf(Float): 8
	sizeOf(String): 16
	sizeOf(Bytes): 24
	sizeOf(List): 24
	sizeOf(Map): 32
	sizeOf(Link): 16

Below is the result on go-merkledag's BenchmarkRoundtrip after
re-generating go-codec-dagpb with this change. Note that the dag-pb
schema contains multiple optional fields, such as strings.

	name         old time/op    new time/op    delta
	Roundtrip-8    4.24µs ± 3%    3.78µs ± 0%  -10.87%  (p=0.004 n=6+5)

	name         old alloc/op   new alloc/op   delta
	Roundtrip-8    6.38kB ± 0%    6.24kB ± 0%   -2.26%  (p=0.002 n=6+6)

	name         old allocs/op  new allocs/op  delta
	Roundtrip-8       103 ± 0%        61 ± 0%  -40.78%  (p=0.002 n=6+6)

Schema typekinds which don't directly map to basicnode prototypes, such
as structs and unions, are left as a TODO for now.

I did not do any measurements to arrive at the magic number of 4x, which
is documented in the code. We might well increase it in the future, with
more careful benchmarking. For now, it seems like a conservative starting
point that should cover all basic types.

Finally, re-generate within this repo.

feat(linksystem): add reification to LinkSystem

add an optional reifier into the link system process. The reason to do this is to capture the link
system itself when reification happens, in case it needs to be put into the node, which is often not
accessible by the time you have a node. another alternative would be to make this specific to
selector traversal, similar to LinkNodePrototypeChooser

Add option to tell link system storage is trusted and we can skip hash on read

Have boolean that specifies whether the storage is trusted. if it is, skip hashing on reads

implement non-dag cbor codec

add non-dag json codec

:'(

This is just documenting our defacto policies already.

Unless we can keep these up to date with go-mod state automatically
(and we don't currently have tools for that), maintaining both these
systems is problematic.

introduce LinkSystem

(It can be hard to intuit this just by reading the code, because some
of the key relevance is actually in *other* functions that might not
be in this repo at all!)

(Some things got better.  Several others in the area still have not.)

Resolves conflicts in go.sum.
(They were conflicts in a textual sense only, but semantically trivial;
application of `go mod tidy` was sufficient.)

I'm still not fully convinced this is always a win for performance,
but as discussed in
https://github.com/ipld/go-ipld-prime/pull/143#discussion_r582335774 ,
there was past evidence, and reproducing that takes work -- so, we're
going to try to be relatively conservative here and keep this logical
branch behavior in place for now.

(The reason this may be worth re-examining is that the change to
hashing interface probably gets rid of one big source of copies.
That doens't answer the holistic performance question alone, though;
it just reopens it.)

Updated the phrasing on relationship to alterative golang libraries.
The state of play there is a bit more advanced than the last time I
updated this readme.

Was also able to add quite a few more statements on distinguishing
features vs alternative libraries.

More discussion of version strategy.  It's defacto what I've been doing
already; now it is documented.

It's small, it's simple, and it's already widely used as part of unixfs.
So there's no reason it shouldn't be part of go-ipld-prime.

The codec is tiny, but has three noteworthy parts: the Encode and Decode
funcs, the cidlink multicodec registration, and the Bytes method
shortcut. Each of these has its own dedicated regression test.

I'm also using this commit to showcase the use of quicktest instead of
go-wish. The result is extremely similar, but with less dot-import
magic. For example, if I remove the Bytes shortcut in Decode:

	--- FAIL: TestDecodeBuffer (0.00s)
	    codec_test.go:115:
	        error:
	          got non-nil error
	        got:
	          e"could not decode raw node: must not call Read"
	        stack:
	          /home/mvdan/src/ipld/codec/raw/codec_test.go:115
	            qt.Assert(t, err, qt.IsNil)

And, make a package which can be imported to register "all" of the
multihashes.  (Or at least all of them that you would've expected
from go-multihash.)

There are also packages that are split roughly per the transitive
dependency it brings in, so you can pick and choose.

This cascaded into more work than I might've expected.
Turns out a handful of the things we have multihash identifiers for
actually *do not* implement the standard hash.Hash contract at all.
For these, I've made small shims.

Test fixtures across the library switch to using sha2-512.
Previously I had written a bunch of them to use sha3 variants,
but since that is not in the standard library, I'm going to move away
from that so as not to re-bloat the transitive dependency tree
just for the tests and examples.

This significantly reworks how linking is handled.

All of the significant operations involved in storing and loading
data are extracted into their own separate features, and the LinkSystem
just composes them.  The big advantage of this is we can now add as
many helper methods to the LinkSystem construct as we want -- whereas
previously, adding methods to the Link interface was a difficult
thing to do, because that interface shows up in a lot of places.

Link is now *just* treated as a data holder -- it doesn't need logic
attached to it directly.  This is much cleaner.

The way we interact with the CID libraries is also different.
We're doing multihash registries ourselves, and breaking our direct
use of the go-multihash library.  The big upside is we're now using
the familiar and standard hash.Hash interface from the golang stdlib.
(And as a bonus, that actually works streamingly; go-mulithash didn't.)
However, this also implies a really big change for downstream users:
we're no longer baking as many hashes into the new multihash registry
by default.