gh-117431: Adapt str.{start,end}swith to use the METH_FASTCALL calling convention

[erlend-aasland]

• Issue: Improve performance of startswith, endswith, count, *find, and *index methods for str, bytes and bytearray #117431

[erlend-aasland]

cc. faster-cpython/ideas#671

[erlend-aasland]

# main
$ ./python.exe -m timeit -s "s = 'abcdef'" "s.startswith('abc')"
5000000 loops, best of 5: 89.1 nsec per loop

# this PR
$ ./python.exe -m timeit -s "s = 'abcdef'" "s.startswith('abc')"
10000000 loops, best of 5: 26.4 nsec per loop

[erlend-aasland]

We'll also want to convert str.find and friend and finally get rid of the parsing helper in strlib. However, that is best left to a follow-up PR (#117468). I prefer small and incremental changes.

[erlend-aasland]

Thanks for the review, @JelleZijlstra and @eendebakpt. I'll hold off merging for a day or two.

[erlend-aasland]

Well, with two core devs approving, I'll go ahead and merge. Thanks for the reviews.

[vstinner]

Nice optimization!