Split out the AST/parser code

[alexcrichton]

In preparation for https://github.com/alexcrichton/wasm-bindgen/issues/18 and https://github.com/alexcrichton/wasm-bindgen/issues/42 @fitzgen and I have been talking about concrete next steps. We think the best way to implement this will be to split out the ast module (and ToTokens implementations) from the wasm-bindgen-macro crate.

A standalone crate with access to the AST will then act as the "IR" for wasm-bindgen. Three new crates, wasm-bindgen-parser-{proc_macro,webidl,typescript} would parse their input into the AST and then generate Rust tokens (a Rust program) from the AST.

@fitzgen is working on the first part of splittng out the ast module into a new crate, and then I'll file issues for new parser crates to exist.

[alexcrichton]

Done in https://github.com/alexcrichton/wasm-bindgen/pull/95