Expose WebAssembly.Instance.exports

[daxpedda]

I was specifically attempting to use #3177 but noticed it was impossible to call __wbindgen_thread_destroy() or get __tls_base and __stack_alloc from inside Rust.

Alternatively we could expose WebAssembly.Instance.prototype.exports directly instead.
EDIT: I decided to go ahead and expose the exports directly. Mainly because there were some targets that were much harder for me to implement exposing the instance for.

Or if we don't want a general-purpose solution like this we could expose __wbindgen_thread_destroy(), __tls_base and __stack_alloc through more intrinsics.

I'm not aware of any downsides.
EDIT: I was thinking that maybe exposing exports directly is worse then exposing the instance, but it seems that you can't do anything else with the instance anyway: https://developer.mozilla.org/en-US/docs/WebAssembly/JavaScript_interface/Instance.

Fixes #2513.

[daxpedda]

@alexcrichton would you mind getting this in before the next release (#3400)?

[alexcrichton]

Sure!

[allsey87]

Quick question: is there any reason why:

wasm_bindgen::exports().js_typeof().as_string().unwrap()

is "object", while:

wasm_bindgen::exports().dyn_into::<js_sys::Object>().unwrap();

panics? From my testing, the js_sys::Object returned by unchecked_into seems to be correct/to work...

[daxpedda]

This is very strange.

It has nothing to do with wasm-bindgen though, I tried it out and apparently Instance.exports returns false on exports instanceof Object, I don't know why.
But typeof exports returns "Object" and the developer console says that it's an Object as well.

No clue!