Remove rtio

This PR includes a sequence of commits that gradually dismantles the `librustrt` `rtio` system -- the main trait previously used to abstract over green and native io. It also largely dismantles `libnative`, moving much of its code into `libstd` and refactoring as it does so.

TL;DR:

* Before this PR: `rustc hello.rs && wc -c hello` produces 715,996
* After this PR:  `rustc hello.rs && wc -c hello` produces 368,100

That is, this PR reduces the footprint of hello world by ~50%.

This is a major step toward #17325 (i.e. toward implementing the [runtime removal RFC](https://github.com/rust-lang/rfcs/pull/230).) What remains is to pull out the scheduling, synchronization and task infrastructure, and to remove `libgreen`. These will be done soon in a follow-up PR.

Part of the work here is eliminating the `rtio` abstraction, which in many cases means bringing the implementation of io closer to the actual API presented in `std::io`.

Another aspect of this PR is the creation of two new, *private* modules within `std` that implement io:

* The `sys` module, which represents a platform-specific implementation of a number of low-level abstractions that are used directly within `std::io` and `std::os`. These "abstractions" are left largely the same as they were in `libnative` (except for the removal of `Arc` in file descriptors), but they are expected to evolve greatly over time. Organizationally, there are `sys/unix/` and `sys/windows/` directories which both implement the entire `sys` module hierarchy; this means that nearly all of the platform-specific code is isolated and you can get a handle on each platform in isolation.

* The `sys_common` module, which is rooted at `sys/common`, and provides a few pieces of private, low-level, but cross-platform functionality.

In the long term, the `sys` modules will provide hooks for exposing high-level platform-specific APIs as part of `libstd`. The first such API will be access to file descriptors from `std::io` abstractions, but a bit of design work remains before that step can be taken. 

The `sys_common` module includes some traits (like `AsFileDesc`) which allow communication of private details between modules in disparate locations in the hierarchy; this helps overcome the relatively simple hierarchical privacy system in Rust.

To emphasize: the organization in `sys` is *very preliminary* and the main goal was to migrate away from `rtio` as quickly and simply as possible. The design will certainly evolve over time, and all of the details are currently private.

Along the way, this PR also entirely removes signal handling, since it was only supported on `librustuv` which was removed a while ago. 

Because of the removal of APIs from `libnative` and `librustrt`, and the removal of signal handling, this is a:

[breaking-change]

Some of these APIs will return in public from from `std` over time.

r? @alexcrichton