Add support for version specifications

[alexcrichton]

This commit adds a #[wasm_bindgen(version = "...")] attribute support. This
information is eventually written into a __wasm_pack_unstable section.
Currently this is a strawman for the proposal in ashleygwilliams/wasm-pack#101

[alexcrichton]

@ashleygwilliams ok I've updated with our discussion this morning:

• Imports that have a module name which starts with ./ cannot have version listed
• All other imports must have a version listed
• The custom section generated for wasm-pack only lists package names (in theory). The module = "..." directives may not be package names due to how imports sometimes work in JS. As a result, some postprocessing of the string specified to module = "..." is done to ensure that wasm-pack only sees package names, not erroneous names like foo/bar/baz
  • Names that don't start with @ assume that everything up to the first / character (if any) is the package name
  • Names that start with @ assume that everything up to the second / character (if any) is the package name

[Pauan]

The example should use #[wasm_bindgen(module = "moment", version = "^2.0.0")]

Unlike Rust, npm does not default to semver (the string "2.0.0" matches the exact version 2.0.0).

So if you want to use semver semantics you need to add a ^ at the start (which then causes it to match anything >=2.0.0 and <3.0.0)

[alexcrichton]

@Pauan ah indeed! That'll happen automatically here, but I can update example usage of this to recommend that. Currently wasm-bindgen doesn't interpret these strings at all, it just ferries them along

[ashleygwilliams]

@alexcrichton @Pauan it's defnitely an interesting question what type of "semver defaults" we want to enforce given that npm and cargo have different ones. seems like a decision for wasm-pack to make, and i'm vaguely leaning towards keeping to the rust defaults in rust and translate those to npm one's just when we move the values to the package.json in the interest of keeping rust people in rust ecosystem defaults. would be curious what others thoughts are here and i'll probably open an issue on wasm pack about it

[alexcrichton]

Ok sounds good!

I think this is ready to go so I'm going to merge.