This commit leverages two new attributes in the Rust compiler,
`#[wasm_custom_section]` and `#[wasm_import_module]`. These two attributes allow
removing a lot of hacks found in wasm-bindgen and also allows removing the
requirement of `wasm-opt` to remove the unused data sections.
This does require two new nightly features but we already required the
`proc_macro` nightly feature and these will hopefully be stabilized before that
feature!
Along the way remove the namespace in Rust as this ended up causing too many
problems, alas! The `js_namespace` attribute now almost exclusively modifies the
JS bindings, hence the "js" in the name now.
