Use the wasm_bindgen_downcast crate for downcasting JsValues

[Michael-F-Bryan]

We need this because @wasmer/wasi gets minified, which breaks the JsValue -> WasmerRuntimeError downcast in the same way that was mentioned in rustwasm/wasm-bindgen#2231 (comment).

The wasm-bindgen-downcast crate works by associating a unique JavaScript Symbol with each Rust type implementing the DowncastJS trait. That gives us something we can check before downcasting a JsValue back to a WasmerRuntimeError.

The underlying issue is that @wasmer/wasi implements the exit() syscall by raising a WasmerRuntimeError with the exit code as a JavaScript exception and catching it from the outside. To get the exit code back, we try to downcast the JsValue we caught back to a WasmerRuntimeError, but because the minifier renamed "WasmerRuntimeError" to "H", the string-based generic_of_jsval() downcast doesn't work.

Using symbols fixes this issue because a symbol is guaranteed to be unique and unchanging once you create it (wasm-bindgen-downcast stores it in a once-cell).

See wasmerio/wasmer-js#310 for a much more in-depth explanation of the issue.

[MTRNord]

Hi is it intentional that https://github.com/wasmerio/wasm-bindgen-downcast is not public? The crates.io page claims it to be MIT and Apache2 licensed however the github repo is not public.