This commit removes the `librustuv` crate.
See the [runtime removal
RFC](https://github.com/rust-lang/rfcs/pull/230) for more context.
See [green-rs](https://github.com/alexcrichton/green-rs/) for a possible
migration path if you wish to continue using green-threaded I/O. The
library provides its own I/O API surface.
[breaking-change]