Recode parser to closer map to web_sys

The parser code I wrote in: https://github.com/jonathanKingston/wurst/blob/master/crates/parser/src/lib.rs reparses webidl when it's basically just trying to wrap the code that web_sys is generating. (both methods `parse` and `method_calls` are responsible for generating the web_sys method names etc)

Currently I am:
- Checking the interfaces
- Getting setter names `set_id`

I tried using the backend of web_sys but most of it is private, having some form of AST which represents the code generation would save people from writing this code.

/cc @fitzgen this is the issue we discussed on irc.