PEP 839: PyFrozenSetWriter and PyFrozenDictWriter C API

Add two builder (“writer”) C APIs, PyFrozenSetWriter and PyFrozenDictWriter, following the design of PyBytesWriter (PEP 782). A writer collects items internally; *_Finish() produces the immutable object — a frozenset or a frozendict (PEP 814) — in a single pass, without ever exposing a mutable intermediate object.