Add constructor property to Object

[RReverser]

Fixes #1361, although shortly after adding this I realised that to be useful wasm-bindgen also needs to expose a way to get constructor of any type as a Function, but not sure what is the best way to expose that yet...

Something like autogenerated fn as_constructor() { ... } on any #[wasm_bindgen] imported type maybe?

Anyway, that can probably be done in a separate PR.

[alexcrichton]

Looks good to me, thanks!

I think that with Deref you can already do Object::constructor(&any_wasm_bindgen_type), right?

[RReverser]

I think that with Deref you can already do Object::constructor(&any_wasm_bindgen_type), right?

Well you can do even any_value.constructor(), yes, but the problem is that you can't do comparisons like any_value.constructor() == Array::constructor_as_a_function_or_something_like_that() because there is no method to get a constructor of a statically defined type.

[RReverser]

Created a separate issue for tracking #1406