* #53: Make a GreenPile with no spawn()s an empty sequence.
Remove the GreenPile.used flag. This was probably originally intended to
address the potential problem of GreenPool.starmap() returning a (still empty)
GreenPile, a problem that has since been addressed by using GreenMap which
requires an explicit marker from the producer. Currently the only effect of
GreenPile.used is to make GreenPile hang when used with an empty sequence.
Test the empty GreenPile case.
Even though GreenMap is probably not intended for general use, make it
slightly more consumer-friendly by adding a done_spawning() method instead of
requiring the consumer to spawn(return_stop_iteration). GreenPool._do_map()
now calls done_spawning(). Remove return_stop_iteration().
Since done_spawning() merely spawns a function that returns StopIteration, any
existing consumer that explicitly does the same will still work. However, this
is potentially a breaking change if any consumers specifically reference
eventlet.greenpool.return_stop_iteration() for that or any other purpose.
Refactor GreenPile.next(), breaking out the bookkeeping detail to new _next()
method. Make subclass GreenMap call base-class _next(), eliminating the need
to replicate that bookkeeping detail.