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!

tests/non-debug.rs

@@ -5,7 +5,7 @@ fn works() {
     test_support::project()
         .debug(false)
         .file("src/lib.rs", r#"
-            #![feature(proc_macro)]
+            #![feature(proc_macro, wasm_custom_section)]
 
             extern crate wasm_bindgen;