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.

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