Deprecate slice __getitem__ and __setitem__ on HumanName #258

The list/dict emulation on HumanName is wider than its value:

• name[1:-3] slices the internal members list positionally — the oddest surviving API (currently a usage.rst example); field access by position has no real use case
• name['first'] = x duplicates plain attribute assignment

Keep: name['first'] string-key access (harmless, useful for dynamic field names), and __iter__/__len__ — len(name) == 0 was enshrined in 1.3.0 as the emptiness check when unparsable was removed, and __str__ iterates for the string_format=None path.

Bridge (1.4) — tracked in #263

• DeprecationWarning on slice __getitem__ and on __setitem__
• Replace the usage.rst slicing example