ioloop: Redefine instance() in terms of current()

ioloop: Redefine instance() in terms of current()

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().