bpo-46311: Clean up PyLong_FromLong and PyLong_FromLongLong

[mdickinson]

PR #27832 inadvertently introduced a couple of changes to PyLong_FromLong that didn't make a lot of sense: an (unsigned long) cast was replaced with (twodigits), and a digit count variable (counting number of PyLong digits in a C long) had its type needlessly changed from int to Py_ssize_t.

• The first change is a potential portability bug, but only on platforms with 128-bit longs. The (unsigned long) cast is obviously correct, while figuring out whether (twodigits) loses information takes some work.
• The second change is merely a potential pessimization: there's no need to use what's typically a 64-bit integer to count the number of PyLong digits in a long.

This PR:

• reverts those two changes
• moves the check for medium values earlier in the function (immediately after the small values check), and simplifies that check
• makes the code a bit less branchy (from experiments locally and on godbolt.org, the expression ival < 0 ? 0U-(unsigned long)ival : ival) gets compiled to something branchless on most platforms, as does ival < 0 ? -ndigits : ndigits
• introduces parallel changes for PyLong_FromLongLong, which now has a fast path for medium-size values.

https://bugs.python.org/issue46311

[erlend-aasland]

I don't see how this qualifies for skip issue. Perhaps I am misreading the devguide, but to me this PR seems inappropriately labelled. If I am mistaken; sorry for the noise.

[mdickinson]

@erlend-aasland Yes, I think you're right. I'll add an issue.

[mdickinson]

This is a minor consistency fix, following the Boy Scout Rule: the prevailing style in this file is to leave a blank line between the comment-description and the definition. But the change is not strictly necessary for this PR, and I can revert if reviewers prefer.

[markshannon]

Thanks. Looks good on initial review.
There are a couple of warnings (lines 294 and 1118) that need fixing.
I re-review once those are fixed.

[mdickinson]

Two changes that should silence compiler warnings.

[mdickinson]

Thanks, @markshannon. The compiler warnings have been silenced.

[markshannon]

Thanks