Remove all use of librustuv
Remove iotest macro

This commit removes the `iotest!` macro from `std::io`. The macro was
primarily used to ensure that all io-related tests were run on both
libnative and libgreen/librustuv. However, now that the librustuv stack
is being removed, the macro is no longer needed.

See the [runtime removal
RFC](https://github.com/rust-lang/rfcs/pull/230) for more context.

[breaking-change]
Remove librustuv

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]
Remove libuv, gyp

This commit removes the libuv and gyp submodules, as well as all build
infrastructure related to them.

For more context, see the [runtime removal
RFC](https://github.com/rust-lang/rfcs/pull/230)

[breaking-change]
Remove std::io::signal

The `std::io::signal` API was only implemented under `librustuv`, which
is now being removed. Rather than keep around an unimplemented API, this
commit removes it altogether.

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 for signal handling code, although in the long run we
plan to add native signal handling to `std::io`.

[breaking-change]
auto merge of #17673 : aturon/rust/remove-uv, r=alexcrichton

This PR begins the process of [runtime removal](https://github.com/rust-lang/rfcs/pull/230) by dismantling the `librustuv` crate and associated event loop.

The result is that, while `libgreen` can still be used for task scheduling purposes, it will no longer be feasible to use green-threaded I/O.

Removing the libuv-based event loop eases the transition away from the runtime system, which will be done incrementally.

In terms of visible API changes, this PR:

* Removes `std::io::signal`, which was never implemented on the native threading model.

* Removes the `iotest!` macro, which was previously used to run I/O tests on both green and native threading models.

* Removes the `green_start!` macro for starting an application with a `librustuv` event loop.

* Removes the `librustuv` crate itself.

It also removes the `libuv` and `gyp` submodules and adjusts the build system and copyright notices accordingly.

If you wish to continue using `librustuv` and green-threaded I/O, consider using [green-rs](https://github.com/alexcrichton/green-rs/), which provides its own I/O stack.