Backend refactor (#411)

* 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.

crates/macro/Cargo.toml

@@ -22,3 +22,4 @@ syn = { version = '0.14', features = ['full'] }
 quote = '0.6'
 proc-macro2 = "0.4.4"
 wasm-bindgen-backend = { path = "../backend", version = "=0.2.11" }
+wasm-bindgen-shared = { path = "../shared", version = "=0.2.11" }