Deep copy Field's default value during Packet's fields initialization.

The Field's default object used to be shared along instances of the Field, ie.:

>>> class A(Packet):
...     fields_desc = [IntField('i', None)]
...
>>> class B(Packet):
...     fields_desc = [PacketListField('a', [], A)]
...
>>> B().a is B().a
True

Fix it, so:
>>> B().a is B().a
False