Wasm-bindgen: Custom `Debug` implementations for js_sys::*

[derekdreery]

It would be nice when debugging if you could get more information about the contents of a javascript object, etc. Debug could be implemented to iterate over its own properties and use f.debug_map to print them.

Same applies to other built-in javascript objects.

[alexcrichton]

This sounds like a great idea to me, and I think we could take a PR for these at any time!

[derekdreery]

I've implemented something in #1161, I'm seeing how it works out in practice and it seems to be good.

It basically tries to json-encode the object, and if that's not possible use toString to get something.

[alexcrichton]

We've already added a few of these so I'm going to close this, and more are of course welcome at any time!