BridgeJS: Support @JS struct on return type of async exported @JS func #753

@JS struct Foo {}
@JS func getFoo() async -> Foo {}

@JSFunction func Promise_resolve_<mangled Foo>(_ promise: JSObject, _ value: Foo) throws(JSException)
@JSFunction func Promise_reject(_ promise: JSObject, _ value: JSValue) throws(JSException)

@_expose(wasm, "bjs_getFoo")
@_cdecl("bjs_getFoo")
public func _bjs_getFoo() -> Int32 {
    #if arch(wasm32)
    return _bjs_makePromise(resolve: Promise_resolve_<mangled Foo> reject: Promise_reject) {
        return await getFoo()
    }
    #else
    fatalError("Only available on WebAssembly")
    #endif
}

@_expose(wasm, "bjs_getFoo")
@_cdecl("bjs_getFoo")
public func _bjs_getFoo() -> Int32 {
    #if arch(wasm32)
    let ret = JSPromise.async {
        return await getFoo().jsValue
    }.jsObject
    return ret.bridgeJSLowerReturn()
    #else
    fatalError("Only available on WebAssembly")
    #endif
}