gh-118750: Asymptotically faster int(string) (#118751)

Asymptotically faster (O(n log n)) str->int for very large strings, leveraging the faster multiplication scheme in the C-coded `_decimal` when available. This is used instead of the current Karatsuba-limited method starting at 2 million digits.

Lots of opportunity remains for fine-tuning. Good targets include changing BYTELIM, and possibly changing the internal output base (from 256 to a higher number of bytes).

Doing this was substantial work, and many of the new lines are actually comments giving correctness proofs. The obvious approaches sticking to integers were too slow to be useful, so this is doing variable-precision decimal floating-point arithmetic. Much faster, but worst-possible rounding errors have to be wholly accounted for, using as little precision as possible.

Special thanks to Serhiy Storchaka for asking many good questions in his code reviews!

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: sstandre <43125375+sstandre@users.noreply.github.com>
Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>
Co-authored-by: Nice Zombies <nineteendo19d0@gmail.com>