GeezerVison

Add primitives for str.ljust(), str.rjust() and str.center() (with and without the fillchar argument), and str.zfill().

The implementation follows pad() and the related methods in CPython's Objects/unicodeobject.c, including:

• the rounding used by center() for odd padding,
• moving a leading +/- sign in zfill(),
• returning the original object if no padding is needed and it's an exact str (a str subclass instance is converted to an exact str),
• validating fillchar even when no padding is needed, with the same error messages.

Tests compare the results against the generic implementation (through Any) for strings and fill characters of all string kinds (ASCII, Latin-1, UCS-2, UCS-4) and a range of widths, including negative widths. Error cases and object identity are also tested.

Micro-benchmark (compiled code, 3.2M calls, best of 15, before -> after, Python 3.13):

Part of mypyc/mypyc#644.