Add a uv backend to core::run

[brson]

When running in a scheduler event loop, core::run should defer to the event loop for running processes. This will avoid blocking waitpid calls.

[alexcrichton]

I'm currently working on this, within the week I should have a pull request or at least a branch to github posted. I'm not redesigning core::run yet, but rather reimplementing the current interface on top of libuv.

[alexcrichton]

After lots of false starts, I believe that this is blocked on joyent/libuv#887 unless we want to concoct a workaround which runs all process events in one and only one event loop.

[alexcrichton]

I've got a temporary patch from one of the node contributors on that issue, and now libuv upgrades and process bindings are on their way soon.

[alexcrichton]

Keeping this updated again, my work is at https://github.com/alexcrichton/rust/tree/issue-6436-run-non-blocking. Tonight I fixed two problems:

1. If you dropped a process that failed to spawn you just deadlocked forever (should have had an assertion preventing this).
2. If you fail to spawn, you still need to call uv_close on the handle you passed into uv_spawn because it doesn't de-initialize it (even though it wasn't initialized beforehand...)

Regardless, now I've almost made it through make check. This gets all the way to running the rpass benchmarks, and then I hit a deadlock again. Will continue investigating, and will try to keep this updated.