Remove rustuv, libuv, and gyp.

As per RFC 62 (https://github.com/rust-lang/rfcs/blob/master/active/0062-remove-runtime.md),
libgreen is in the process of being removed from Rust.

To begin this process, the I/O support in libuv and librustuv is being removed. libgreen will
remain in a transitional state so that users of the library will have time to
adapt to the removal.

[breaking-change]