Implement a basic event loop without libuv

This is more progress towards #9128 and all its related tree of issues. This implements a new `BasicLoop` on top of pthreads synchronization primitives (wrapped in `LittleLock`). This also removes the wonky `callback_ms` function from the interface of the event loop.

After #9901 is taking forever to land, I'm going to try to do all this runtime work in much smaller chunks than before. Right now this will not work unless #9901 lands first, but I'm close to landing it (hopefully), and I wanted to go ahead and get this reviewed before throwing it at bors later on down the road.

This "pausible idle callback" is also a bit of a weird idea, but it wasn't as difficult to implement as callback_ms so I'm more semi-ok with it.