c-api: component-model: Lookup function from an instance

[MangoPeachGrape]

Only get_func() as of now, should #9812 (comment) come in a later PR?

One thing of note, if name isn't UTF-8, it returns false instead of returning an error, is that fine?

[alexcrichton]

Thanks! This has triggered some further thoughts which are both preexisting and related to this PR as well:

• For names I realize now we shouldn't use const char*, but instead we should be using wasm_name_t*. Unicode can't be represented as const char* due to the lack of ability to support interior nul characters, so we'll want to change this and other APIs to use wasm_name_t instead.
• For this I think we'll want to base lookup of an item on an ComponentExportIndex instead of a string, although a string can be used to acquire an ComponentExportIndex. Effectively I think we should bind {Instance,Component}::get_export_index and then an export index is used to lookup a function. That'll handle nested instances as well as top-level exports in the same manner.

[alexcrichton]

Er, sent too soon:

One thing of note, if name isn't UTF-8, it returns false instead of returning an error, is that fine?

Yeah sounds good!

---

One other slightly orthogonal thing, I think it'd be nice to help clean up the C tests a bit. If you'd like I think it'd be reasonable to export the C API implementation pointer from C++ structures, that way you could sort of intermix C++ and C where the C++ APIs could be used to handle auto-destruction and such. That might also make it easier to bind optional errors and assert that errors don't happen. Otherwise could the CHECK macro be deduplicated in a header instead of duplicated between files?

[MangoPeachGrape]

Next up values, which are required for function invocation?

[alexcrichton]

Yeah that seems reasonable. I'll be honest though in that I don't really know how to do values. Everything I've thought of historically is either extremely chatty over the C API boundary or extremely allocation-heavy, neither of which I feel is great... If you've got ideas though please dive in!