In addition to closing #495 this'll be useful eventually when instantiating
multiple wasm modules from Rust as you'd now be able to acquire a reference to
the current module in Rust itself.
First added in #161 this never ended up panning out, so let's remove the
experimental suport which isn't actually used by anything today and hold off on
any other changes until an RFC happens.
This commit starts migrating the `wasm_bindgen` tests to the `wasm_bindgen_test`
framework, starting to assemble the coffin for
`wasm-bindgen-test-project-builder`. Over time all of the tests in
`tests/all/*.rs` should be migrated to `wasm_bindgen_test`, although they may
not all want to go into a monolithic test suite so we can continue to test for
some more subtle situations with `#[wasm_bindgen]`.
In the meantime those, the `tests/all/api.rs` tests can certainly migrate!