GitLab

Outline of Traverse* funcs; comment about link loaders in TraversalConfig; confession of working draft of Selector interface; fixed typo in AdvVisitFn return types.

Yes, that's lot of things heaped in one commit.

Yes, (unfortunately,) they're all related.  Almost the entirety of this
has to manifest *at once*, all seamlessly connected, for any part of
it to get off the ground.

Note how very much is handwaved in the comments about TraversalConfig
having linkloader tools of some kind.  There's a lot to unpack there.
Getting it to work well, and let users of the library supply functions
that customize the parts that are interesting to the user, while *not*
getting them hamstrung by a bunch of details about multicodecs and
multihashing (unless they want to!)... requires careful design work.
Signed-off-by: Eric Myhre <hash@exultant.us>