GitLab

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>

It covers the bases on reading, and has a NodeBuilder which works to
create new nodes based on the type-level structure.

(Quite a few other things about it are incomplete, and it might end
up getting merged this way, because the goal here was primarily to
scope out if any of our abstractions will shatter badly, and we've
now got information on that.)

This seems to indicate we are indeed on the right track, regarding
some of those comments in the codegeneration work in the previous
commit: check out the 'Representation' method.  Yep -- we've got
multiple nodebuilders in our future.
Signed-off-by: Eric Myhre <hash@exultant.us>