Runtime removal: add private sys, sys_common modules

These modules will house the code that used to be part of the runtime system
in libnative. The `sys_common` module contains a few low-level but
cross-platform details. The `sys` module is set up using `#[cfg()]` to
include either a unix or windows implementation of a common API
surface. This API surface is *not* exported directly in `libstd`, but is
instead used to bulid `std::os` and `std::io`.

Ultimately, the low-level details in `sys` will be exposed in a
controlled way through a separate platform-specific surface, but that
setup is not part of this patch.
Remove signal handling.

Since signal handling was only implemented through librustuv, which is
now gone, there's no reason to even provide the API.
Runtime removal: Refactor fs

This moves the filesystem implementation from libnative into the new
`sys` modules, refactoring along the way and hooking into `std::io::fs`.
Runtime removal: refactor pipes and networking

This patch continues the runtime removal by moving pipe and
networking-related code into `sys`.
Runtime removal: refactor helper threads

This patch continues the runtime removal by moving
libnative::io::helper_thread into sys::helper_signal and
sys_common::helper_thread
Runtime removal: refactor process

This patch continues the runtime removal by moving and refactoring the
process implementation into the new `sys` module.
Runtime removal: refactor timer

This patch continues runtime removal by moving out timer-related code
into `sys`.
Runtime removal: refactor tty

This patch continues runtime removal by moving the tty implementations
into `sys`.
Runtime removal: rully remove rtio

This patch cleans up the remnants of the runtime IO interface.