This commit adds an option to "uglify" the wasm module's imports/exports so
those which are controlled by bindgen are renamed to a shorter (probably one
letter) names. This'll hopefully help cut down on both the wasm size slightly
and also the generated JS as the glue we're talking to wasm over won't require
such large names all the time.
* Use a bundled custom `WasmRefCell` instead of the one in the standard library.
This one primarily doesn't panic via libstd which means that its code
footprint is much smaller.
* Add a `throw` function to `wasm_bindgen`-the-crate which can be used to throw
an exception in JS from Rust. This is useful as a cheap way to throw an
exception code-wise (little code bloat) and it's also a great way of reporting
error messages to JS!
* Cut down on the code size of `__wbindgen_malloc` by aborting on huge requests
earlier.
* Use a custom `assert_not_null` function which delegates to `throw` to test for
incoming null pointers
