[WIP] Implement window.fetch

[thgh]

I commented src/handlers.rs#L183 to keep it compiling. Otherwise I typically get these errors:

error[E0373]: closure may outlive the current function, but it borrows `id`, which is owned by the current function
   --> ../../src/handlers.rs:182:10
    |
182 |     .map(|res| {
    |          ^^^^^ may outlive borrowed value `id`

error[E0277]: `*const binding::DenoC` cannot be shared between threads safely
   --> ../../src/handlers.rs:168:3
    |
168 |   rt::run(req);
    |   ^^^^^^^ `*const binding::DenoC` cannot be shared between threads safely

[ry]

Sorry for the delay in reviewing! I will get to this soon - we're almost caught up.

[ry]

Thanks @thgh ! I'm continuing to work on this in #512 where i've rebased your commits. Closing this one.