feat: add entity matcher w/o preload, add matcher fn for consuming bytes

[rvagg]

Draft, WIP, for experimenting with filecoin-project/lassie#306

• Added MatchUnixFSEntitySelector as a replacement for MatchUnixFSPreloadSelector that doesn't use unixfs-preload. Added a possible deprecation notice to MatchUnixFSPreloadSelector.
• Added BytesConsumingMatcher as a matcher function for traversal.WalkMatching that will slurp bytes for you upon match, designed for use with the Match* selectors in here.

[rvagg]

FYI, this .. works, replacing unixfs-preload

	return ssb.ExploreInterpretAs("unixfs-preload", ssb.Matcher())

with

	return ssb.ExploreInterpretAs("unixfs", ssb.ExploreUnion(ssb.Matcher(),
		ssb.ExploreRecursive(
			selector.RecursionLimitDepth(1),
			ssb.ExploreAll(ssb.ExploreRecursiveEdge()),
		),
	))

and consuming bytes in a matcher.

But I think it fails for the non-unixfs case where we want to match "entity". I wonder was that why this approach wasn't taken with the original request that sparked unixfs-preload (filecoin data explorer IIRC)? Or was there something else I'm missing?

[willscott]

My explanation here is that preload when implemented was focused almost entirely on the directory case rather than on the file case.

[hannahhoward]

@rvagg doesn't this still traverse the dir links? It's not the same no? Therefore it traverses more than the required bytes?

[rvagg]

Ready for review now, updated OP with this:

• Added MatchUnixFSEntitySelector as a replacement for MatchUnixFSPreloadSelector that doesn't use unixfs-preload. Added a possible deprecation notice to MatchUnixFSPreloadSelector.
• Added BytesConsumingMatcher as a matcher function for traversal.WalkMatching that will slurp bytes for you upon match, designed for use with the Match* selectors in here.

[hannahhoward]

LGTM