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As discussed on the issue thread, ipld.Kind and schema.TypeKind are more
intuitive, closer to the spec wording, and just generally better in the
long run.

The changes are almost entirely automated via the commands below. Very
minor changes were needed in some of the generators, and then gofmt.

	sed -ri 's/\<Kind\(\)/TypeKind()/g' **/*.go
	git checkout fluent # since it uses reflect.Value.Kind

	sed -ri 's/\<Kind_/TypeKind_/g' **/*.go
	sed -i 's/\<Kind\>/TypeKind/g' **/*.go
	sed -i 's/ReprKind/Kind/g' **/*.go

Plus manually undoing a few renames, as per Eric's review.

Fixes #94.

See the changelog for discussion; this had already been on the docket
for a while now.

ipld.Path is now a slice of ipld.PathSegment instead of strings.

This should be an improvement in sanity: there are now several fewer
places importing "strconv", and that's just always a good thing.

We will also be free in the future to add PathSegment-based accessor
methods to ipld.Node, as has already been commented as a desire;
and, to use PathSegment in building better typed errors
(which is the specific thing that provokes this diff today and now).

The implementation of PathSegment is now also based on a struct-style
union rather than an interface style union.  There are comments about
this in the diff.  I do not wish to comment on how much time I've spent
looking at golang assembler and runtime internals while trying to find
a path to a more perfect compromise between ergonomics and performance.
tl;dr Selectors will probably get faster and trigger fewer allocations;
ipld.Path will probably take slightly more memory (another word per
path segment), but not enough to care about for any practical purposes.

I did not attempt to hoist the SegmentIterator features from the
selector package to anywhere more central.
It may be a fine idea to do so someday; I just don't presently have
a formed opinion and am not budgeting time to consider it today.
Signed-off-by: Eric Myhre <hash@exultant.us>

Most important things first!  To follow this refactor:

```
sed s/TraverseField/LookupString/g
sed s/TraverseIndex/LookupIndex/g
```

It is *literally* a sed-refactor in complexity.

---

Now, details.

This has been pending for a while, and there is some discussion in
https://github.com/ipld/go-ipld-prime/issues/22 .

In short, "Traversal" seemed like a mouthful;
"Field" was always a misnomer here;
and we've discovered several other methods that we *should* have
in the area as well, which necessitated a thought about placement.

In this commit, only the renames are applied, but you can also see
the outlines of two new methods in the Node interface, as comments.
These will be added in future commits.
Signed-off-by: Eric Myhre <hash@exultant.us>

Extract the selector buidler to a seperate package

Rename SelectorContext to ParsedParent (more accurate) and put in a ParseContext state object which
is tracked w/o var args

Support validating recursive selectors to match ExploreRecursive with ExploreRecursiveEdge

Adds unit test of each type of selector to check parsing and functionality. Also includes some fixes
discovered while creating unit tests

- Update existing selectors so they are current with the specification.
- Also add early implementations of ExploreIndex and ExploreRange.
- Add an early implementation of Explore for ExploreUnion.
- Add constants for field keys.