This aligns us more closely with asyncio, which only has a single
get_event_loop() function. Most uses of instance() are simply a
holdover from before current() was introduced and are not actually
relying on its slightly different behavior, so we redefine it (and the
related methods install(), initialized(), and clear_instance()) in
terms of current().
call_later is a less-verbose alternative to add_timeout with a
timedelta; call_at exists for symmetry. Both are named after
methods on the asyncio event loop, although there are small
variations (we support both args and kwargs while asyncio only supports
args; we use remove_timeout(handle) instead of handle.cancel()).
Closes#1049.
Ben Darnell