gh-125916: convert reduce() to Argument Clinic

[skirpichev]

• Issue: Allow functools.reduces 'initial' to be a keyword argument #125916

[skirpichev]

This coming from #125917. Red alert: to make AC happy, docstring was adjusted.

I did some benchmarks.

# a.py
import pyperf
from functools import reduce

f = lambda x, y: x + y
lst = list(range(5))
init = 123

runner = pyperf.Runner()
runner.bench_func('reduce(f, lst)', reduce, f, lst)
runner.bench_func('reduce(f, lst, init)', reduce, f, lst, init)

Run e.g. with: python a.py -q -o ref.json.

with results:

Benchmark	ref	patch
reduce(f, lst)	1.27 us	1.19 us: 1.07x faster
reduce(f, lst, init)	1.43 us	1.36 us: 1.05x faster
Geometric mean	(ref)	1.06x faster

So, at least AC-backed argument processing offer some speedup.

[erlend-aasland]

Thanks Sergey, I'll review this within today. Ping me tomorrow if I forget.