Put uv in its own crate

[brson]

For #4419 we want uv to be the default event loop implementation, but do not want to put it into core. Instead it will live in a uv crate with a dependency on core and implementing several types in core. When generating the crate entry point, rustc will pass rust_start a pointer to a factory function that returns the uv loop.

[emberian]

Still relevant