State defaults and factories

[JamesArruda]

States currently handle mutable default values by deepcopying everything. Even with default_factory, the factory function gets called once and the result is deepcopied.

This means tests like this pass:

class Cashier(UP.Actor):
    items_scanned = UP.State[list](default=[])

c1 = Cashier(...)
c2 = Cashier(...)
assert id(c1.items_scanned) != id(c2.items_scanned)

when you might expect that to be the behavior.

This is acceptable for now, as the resulting behavior is what one would expect or desire.

If a default factory is supposed to be random, or expected to do anything more than return empty mutable objects, then this ticket will be necessary.