Enable missing_docs errors in the wasm crate.

This adds `#![deny(missing_docs)]` to the wasm crate, and adds documentation
to several struct and enum fields, as needed.

lib/wasm/src/lib.rs

@@ -10,6 +10,8 @@
 //!
 //! The main function of this module is [`translate_module`](fn.translate_module.html).
 
+#![deny(missing_docs)]
+
 extern crate wasmparser;
 extern crate cton_frontend;
 extern crate cretonne;