This commit implements a system that will assert that all
`#[wasm_bindgen]` attributes are actually used during compilation. This
should help ensure that we don't sneak in stray attributes that don't
actually end up having any meaning, and hopefully make it a bit easier
to learn `#[wasm_bindgen]`!
First added in #161 this never ended up panning out, so let's remove the
experimental suport which isn't actually used by anything today and hold off on
any other changes until an RFC happens.
