HumanName loses CONSTANTS singleton identity on pickle/deepcopy (has_own_config flips, pickle bloat) #169

Summary

A default HumanName shares the module-level CONSTANTS singleton as its .C.
pickle and copy.deepcopy cannot preserve object identity, so after a
round-trip .C is a detached copy rather than the singleton. This flips
has_own_config from False to True and bloats every serialized default name
with a full Constants copy (~1000+ entries).

Surfaced during the round-trip work in #168 (which fixes the value-level
round-trip bugs but deliberately leaves this identity/semantic issue out of
scope).

Reproduction

import pickle, copy
from nameparser import HumanName
from nameparser.config import CONSTANTS

hn = HumanName("John Doe")
print(hn.has_own_config, hn.C is CONSTANTS)              # False True

r = pickle.loads(pickle.dumps(hn))
print(r.has_own_config, r.C is CONSTANTS)                # True  False

d = copy.deepcopy(hn)
print(d.has_own_config, d.C is CONSTANTS)                # True  False

Impact

• has_own_config reports True for a name that was using the shared config —
  the property effectively lies after a round-trip.
• The restored name no longer tracks later changes to the global CONSTANTS.
• Each pickled/deep-copied default name carries a full Constants copy instead
  of a reference. Real memory/size cost when serializing names in bulk.

This is pre-existing (the old serialization also copied .C by value); #168 did
not introduce it.

Proposed fix

Add HumanName.__getstate__/__setstate__ (or __reduce__) that special-case
self.C is CONSTANTS and restore the reference instead of a copy. Sketch:

def __getstate__(self):
    state = self.__dict__.copy()
    if self.C is CONSTANTS:
        state['C'] = None        # sentinel: shared module config
    return state

def __setstate__(self, state):
    if state.get('C') is None:
        state['C'] = CONSTANTS
    self.__dict__.update(state)

(self.C is never legitimately None — the constructor replaces None with a
fresh Constants — so None is a safe sentinel. Confirm before relying on it.)

Notes / things to decide

• This changes HumanName serialization semantics, so it warrants its own PR and
  a changelog note.
• Needs tests: default name round-trips with .C is CONSTANTS and
  has_own_config == False; an instance-config name still round-trips its own
  config (covered today by the tests added in Fix pickle/copy round-trip bugs in Constants and config managers #168); both pickle and
  copy.deepcopy.
• Decide whether copy.copy (shallow) behavior should match.