Add Windows Support

[mitsuhiko]

There are currently two blockers for Windows support:

• ipc-channel: Windows servo/ipc-channel#233
• findshlibs: Windows support gimli-rs/findshlibs#37 (for the safe-shared-libraries feature)

[pwnorbitals]

I believe those blockers have been lifted now !

[mitsuhiko]

@pwnorbitals would be happy to accept PRs for this. It would take me a bit to get a working windows set up at the moment.

[davehorner]

tried using it today and received the following:

error[E0425]: cannot find function `kill` in crate `libc`
   --> C:\Users\dhorner\.cargo\registry\src\index.crates.io-6f17d22bba15001f\procspawn-0.10.2\src\proc.rs:334:27
    |
334 |                     libc::kill(pid as i32, libc::SIGKILL);
    |                           ^^^^ not found in `libc`

error[E0425]: cannot find value `SIGKILL` in crate `libc`
   --> C:\Users\dhorner\.cargo\registry\src\index.crates.io-6f17d22bba15001f\procspawn-0.10.2\src\proc.rs:334:50
    |
334 |                     libc::kill(pid as i32, libc::SIGKILL);
    |                                                  ^^^^^^^ help: a constant with a similar name exists: `SIGILL`
    |
   ::: C:\Users\dhorner\.cargo\registry\src\index.crates.io-6f17d22bba15001f\libc-0.2.144\src\windows\mod.rs:243:1
    |
243 | pub const SIGILL: ::c_int = 4;
    | ------------------------- similarly named constant `SIGILL` defined here