Rewrite _Node piclking protocol
Rewrite _Node piclking protocol

Since _Node.__getstate__ used recursion (not directly, but the pickling
would recurse to the children dictionary and nodes there) it would fail
for deep tries.  Rewrite the whole thing to be more like iterate method.

Reported-by: Philippe Ombredanne <ca95c4a6a4931f366cbdaf5878c5016609417d37@gmail.com>  # https://github.com/google/pygtrie/issues/9

pombredanne/pygtrie,google/pygtrie