S-expression parsing in C API and binaryen.js

[kripken]

BinaryenModuleParse and Binaryen.parseText respectively.

[dcodeIO]

Is there a chance to support this on function / source code level, too? For example, I have a couple of builtins that I am currently declaring programmatically, but it would be super convenient to be able to use some sort of inline-assembly instead. More a nice-to-have, not super important, of course.

Could be that it is already possible to emulate this somehow with this PR when using temporary modules. Iirc, there is already some debugging utility that does the inverse, i.e. converting a subset of statements or expressions to s-expr syntax. Not sure if that's exposed already, though.

[kripken]

Yeah, this PR gets us pretty close to that. I think we'd just need to add module merging to get what you want. We already have wasm-merge as a tool, it could be refactored into a C API call.