Not a lot of attention has been paid to dealing with conflicts of symbols
between crates and different `#[wasm_bindgen]` blocks. This commit starts to fix
this issue by unblocking #486 which first ran into this. Currently there's a bug
where if two independent crates bind the same JS API they'll generate the same
symbol which causes conflicts for things like LTO or linking in general.
This commit starts to add a "salt" to all symbols generated by `wasm-bindgen`
(these are all transparent to the user) to ensure that each crate's invocations
are kept apart from one another and using the correct bindings.
* remove BindgenAttrs from other backend::ast structs
This is primarily a tool for use with the macro crate. Most of
these attributes were ignored in the actual codegen, but a few
were still being used. This is confusing when trying to add
other sources for codegen (such as webidl and typescript).
* move parsing logic to macro crate
This makes the backend crate solely concerned with having an ast
for which we can generate code.
