[mypyc] Add support for `str.lower()` and `str.upper()` (#20948)
Fixes https://github.com/mypyc/mypyc/issues/1088
Follow up on https://github.com/python/mypy/pull/19375 with full Unicode
support:
- ASCII fast path
- Shared `CPyStr_ChangeCase` helper, parameterized by function pointers
- **`_PyUnicode_ToLowerFull`/`_PyUnicode_ToUpperFull`** for Unicode
which handle 1-to-N expansion (e.g., `ß`→`SS`); This was a sticky point
with the previous PR which relied on `Py_UNICODE_TOLOWER`/`TOUPPER`.
- Temporary `len * 3` UCS-4 buffer for the Unicode path; This is because
each Unicode char may be expanded from 1 byte to 3
