rustuv: Add no-loop constructors

Split constructors so there is both an EventLoop and a non-EventLoop variant.
This also adds a new experimental dependency of a scope tls library which is
wired to native threads (slated for addition into libstd hopefully).
rustuv: Bring the QueuePool back online

Mostly just cosmetic changes, nothing major
rustuv: Re-enable homing
rustuv: Reimplement timers