Implement full bindings for v8::Platform

[mmastrac]

We are likely going to need to create a custom v8::Platform in the long term, so getting a start on this is going to be important. We prototyped mapping C++ vtables to Rust in #1275 and this would be a natural extension of that work, though it could potentially be more brittle.

It might make sense for us just to expose the task management APIs for now, but this may require investigation.

cc @aapoalas

[aapoalas]

Are you reading my mind?! I've had this on my mind the last few weeks and thought I'd surprise the lot of ya by suddenly busting out a complete v8::Platform subclassing solution in Rust.