gh-82088: Improve performance of PyLong_As*() for multi-digit ints

[eendebakpt]

This is a continuation of the work in #15363 by @sir-sigurd.

Performance is improved by

• Unrolling the part of the loop where no overflow can occur
• Improving the overflow check so no temporary variable is used

Performance: main

pack n dt 460.21 [ms]
pack N dt 476.32 [ms]
pack l dt 479.76 [ms]
pack L dt 451.84 [ms]

PR

pack n dt 401.02 [ms]
pack N dt 391.06 [ms]
pack l dt 408.83 [ms]
pack L dt 402.37 [ms]

Script:

import time
from struct import Struct

if __name__ == "__main__":
    for key in 'nNlL':
        N = 1000
        pack = Struct(key*N).pack
        values = tuple(range(2**30, 2**30 + N))
            
        t0=time.perf_counter()
        for _ in range(50_000):
            pack(*values);
        dt=time.perf_counter()-t0
        
        print(f'pack {key} dt {dt*1e3:.2f} [ms]')

(performance gain is similar to earlier reported improvements)

• Issue: speed-up PyLong_As*() for large longs #82088

[eendebakpt]

We can also unroll the second digit for the other methods. It gains a bit of performance at the cost of a bit of extra code. The unrolling can be factored out to a helper function (the loop itself is not so easy because there are small differences in how to handle the overflow)