Remove extra::uv, net, timer, etc. and several obsolete runtime features.

This removes all the code from libextra that depends on libuv. After that it removes three runtime features that existed to support the global uv loop: weak tasks, runtime-global variables, and at_exit handlers.

The networking code doesn't have many users besides servo, so shouldn't have much fallout. The timer code though is useful and will probably break out-of-tree code until the new scheduler lands, but I expect that to be soon.

It also incidentally moves `os::change_dir_locked` to `std::unstable`. This is a function used by test cases to avoid cwd races and in my opinion shouldn't be public (#7870).

Closes #7251 and #7870