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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.