This commit implements callback interfaces for WebIDL, the final WebIDL
construct that we were unconditionally ignoring! Callback interfaces are
implemented as dictionaries of callbacks. Single-operation callback interfaces
are also expanded when flattening to accept a `Function` as well, in accordance
with the WebIDL spec.
New features have been added to `web-sys` for all the new callback interface
types. Additionally the `EventTarget.webidl` was tweaked to not have
`EventListener?` as this is required for all functional usage and there's no
need to keep that sort of web browser compat here.
Closes#258
