bpo-41972: Use the two-way algorithm for string searching (GH-22904)

bpo-41972: Use the two-way algorithm for string searching (GH-22904)

Implement an enhanced variant of Crochemore and Perrin's Two-Way string searching algorithm, which reduces worst-case time from quadratic (the product of the string and pattern lengths) to linear. This applies to forward searches (like``find``, ``index``, ``replace``); the algorithm for reverse searches (like ``rfind``) is not changed.

Co-authored-by: Tim Peters <tim.peters@gmail.com>