Leverage new rustc wasm features

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!

examples/wasm-in-wasm/src/lib.rs

@@ -1,4 +1,4 @@
-#![feature(proc_macro)]
+#![feature(proc_macro, wasm_custom_section, wasm_import_module)]
 
 extern crate wasm_bindgen;