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This is a somewhat altered schema-schema document, as commented.
Some of those alterations might get upstreamed; I'll be making other
PRs to the upstream specs repo to discuss that.

- it now uses kinded unions (since those are supported now; they
  weren't yet the last time we touched this demo!).

- a few typos are fixed (field names; in one case, a place that must
  support type names as well as inline defns).

- added the 'Schema' type at the very top!  this is what parsing the
  schema-schema starts with!

Update tests for unions; several fixes

This is probably now one of the nicer (directly-exercised) examples we
have for how the type-level and representation-level views of data can
differ with the use of schemas.

Interesting to note that this causes the go-wish library to drop out of
the imports list entirely.

The increased coverage found an infinite loop in iterators, so the
fix for this is included in this commit.

New testcase system for exercising typed nodes; Revamp struct tests to use it.

Though in general, letting golang's test system replace spaces and
other unusual characters with underscores has very little downside,
here, the resulting "_--_" is quite unpleasant on the eyes.

Change things to use TitleCase.

This new system focuses on table-driven tests, and leans heavily upon
json as a shorthand for expressing fixtures.

It also makes a great deal more effort to exercise the different
features of nodes (and their paired representation nodes) from all
directions at once for each test datum, rather than requring that all
be written out manually.

The result is that the struct tests we've renovated have a lovely
diffstat shrinkage: 111 insertions, 299 deletions...

And yet the smaller line count results in *more* coverage.

(Okay, the linecount increase for the testcase structure and helper
methods is much bigger than the savings in fixture size... but,
only *so far*.  I assume this will continue to pay off in the future.)

Relatedly: a bug in struct map representations has been fixed.
(It was the sibling of 5f589653, embarassingly.)  Thank goodness we now
get proper coverage of this area.

There's a few TODOs left to further expand the exercises, but those
can slot in easily in subsequent commits.  Same goes for further
expansion of usage of this new system.

CI: switch from Travis to Actions

They're actively maintained, faster on Mac and Windows, and generally
a better mechanism long-term.

The only change applied here, in practice, is that we now test on Mac
and Windows as well.

(This type isn't really "embedded" in the others directly, so those
docs were somewhat confusing.)

More code sections.  Parts of this paragraph were being rendered as
italics.

fix(cidlink): check for byte buffer

Use interface rather than concrete type for byte buffer checker

CI: test with Go 1.14 and 1.15, drop 1.12

The common policy is to support the two latest versions of Go. Do that,
since our previous state meant a version of Go that's pretty old and
unused at this point.

Also simplify the travis yaml. First, GO111MODULE=on hasn't been needed
for a while, since it defaults to "on" if go.mod exists. We also don't
need redundant steps like "go mod download" or "go build"; "go test"
already does all of that for us. The "go fmt" check is also pointless,
as it only formats the code without actually complaining if it isn't
properly formatted.

Finally, update the Go version in go.mod to 1.14, since that's the
oldest version we test against and support at this point. This version
sets, for example, what language features we can use.

And, while at it, "go mod tidy".

linking/cid: check a previously unused error

all: make 'go test ./...' pass on Go 1.15

...to actually run tests.

Ow.

(You can tell I don't really look to travis for meaningful results.)

I am embarrassed to say I ripped out the *actual* running of tests
back during 804a1336, when cleaning
up codecov tools.

Thanks to @mvdan for looking at the CI and CI config!

As spotted by staticcheck. While at it, remove punctuation from another
couple of errors, as per
https://github.com/golang/go/wiki/CodeReviewComments#error-strings:

	Error strings should not be capitalized (unless beginning with
	proper nouns or acronyms) or end with punctuation, since they
	are usually printed following other context.

There were two vet errors in two packages containing tests, resulting in
'go test' erroring out before any tests were run. Both were due to the
same reason - an Error method that ends up calling itself forever, thus
a panic.

While at it, 'gofmt -w -s' everything, which removes a redundant type.

Add traversal.Get function

(This should've been committed earlier.  I have too much WIP standing
uncommitted in my working tree.  Ow.  Mea cupla.)

This doesn't add much in the way of coverage, since it's internally
quickly convergenced with what the Focus tests cover, but in testing,
explicitness is good.

Kinded union gen

The AssignNode method is now supplied; and the previous hacky shrug
regarding switch statements with no applicable cases also had to go.
(Turns out that the no-applicable-cases thing coincidentally worked
for the "embedall" internal implementation mode; but didn't fly for
the "interface" internal style, because that would end up with
variable declarations made but not referenced.  Harrumph.)

This template munging business got nastier as time proceded.
I think we could drastically reduce the amount of redundancy in the
arugments to `kindedUnionNodeMethodTemplateMunge` if we re-did that
function with a more structural understanding of what's going on,
and made some basic understanding of what zero values are per golang
type, etc.

That can be future work, though.  Today, I want working kinded unions,
and I just want them done enough to use; kludge tolerance high.

So far, the generation itself runs, but the result will not yet compile.
I just want a checkpoint here.  (Most of this was written a few days ago already.)

Struct tuple representation codegen