* 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
