Add initial support for compact import section. NFC

[sbc100]

See https://github.com/WebAssembly/compact-import-section

For this initial commit I've just added a single simple test in form of test/binary/compact-imports.txt. Once we update the testsuite repo we can pull in the official tests.

[bvisness]

All looks reasonable to me, although I've not looked at WABT's code before.

[aheejin]

LGTM % few nits

[dschuff]

This does sort of raise the question of how a tool should handle compact imports. Since it looks like there will be both a text and a binary format, you could argue that a true "pass-through/roundtrip" encoding would only encode compact imports in binary where they are encoded in the text. But of course it also makes sense to encode them everywhere possible, just to reduce binary size. Maybe wabt should default to only round-tripping? (And optionally have a flag to opportunistically use compact imports?)

[sbc100]

This does sort of raise the question of how a tool should handle compact imports. Since it looks like there will be both a text and a binary format, you could argue that a true "pass-through/roundtrip" encoding would only encode compact imports in binary where they are encoded in the text. But of course it also makes sense to encode them everywhere possible, just to reduce binary size. Maybe wabt should default to only round-tripping? (And optionally have a flag to opportunistically use compact imports?)

Right now that flag is --enable-compact-imports. Do you think we would ever want the binaryen writer to not opportunistically use compact imports when the feature is enabled?

I guess if we might have users who want to enable them in the reader but disabled them in the writer, but so far that hasn't come up for other features.

[dschuff]

Right, for Binaryen, I don't know of any case where we wouldn't want to opportunistically use compact imports when they are enabled. Wabt is more interesting, because its job is to be a low-level tool that round-trips text and binary, rather than an optimization/transformation tool, and changing the encoding is a transformation.

[sbc100]

Yeah, right no the compact imports are not remembered on read, and we write them purely opportunistically. We should probably followup by preserving them in the IR instead.