extra: Remove uv, net, timer code

This will all be reimplemented in the new runtime.
extra: Add url module

Used to be under extra::net, which no longer exists
std: Remove weak_task API. Unused
std::rt: Stop using unstable::global in change_dir_locked
std: Move change_dir_locked to unstable. #7870
std: Remove unstable::global. Unused
std: Remove at_exit API. Unused
auto merge of #7883 : brson/rust/rm-std-net, r=graydon

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