Remove signal handling.

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

[breaking-change]
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.