This commit migrates from `wasm_bindgen!`-the-macro to
`#[wasm_bindgen]`-the-attribute. The actual mechanics of the macro are
relatively simple in just generating some shims here and there, but wrapping
everything in one huge macro invocation can often seem intimidating as it gives
off this feeling of "oh dear anything can happen here!" Using an attribute
should curb expectations much more greatly of "oh there's just some extra stuff
happening behind the scenes".
The usage is otherwise relatively straightforward and close to what it was
before, but check out the DESIGN.md/README.md changes for more info!
Push the compiler to do trait resolution to figure out what each type is bound
with in JS, and that way we can accept effectively all types (so long as they
implement a trait).
* 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
