[FEATURE] Speeding up JSON and pickle: to_dict() method

[Marco-Sulla]

Currently, if you want to serialize a frozendict to JSON, you have to convert it to dict, even if frozendict is a Mapping. This is a limitation of json builtin and third party modules.

To mitigate speed problems, a to_dict() method can be added. It will return a memcopy of frozendict into a new dict object. It should be much faster than dict(a_frozendict).

This new method should also speed up pickle.dumps(). Since frozendict can be really useful in multiprocessing, this could be a nice bonus.