std.numeric: Always use Stein's algorithm for integral gcd()

[ibuclaw]

Whatever tests/observations were done in #4940 no longer apply when I run benchmarks locally:

ubyte_euclid: 10 secs, 741 ms, 853 μs, and 9 hnsecs
ubyte_stein: 6 secs, 98 ms, 939 μs, and 7 hnsecs
ushort_euclid: 13 secs, 442 ms, 495 μs, and 6 hnsecs
ushort_stein: 9 secs, 266 ms, 581 μs, and 2 hnsecs
uint_euclid: 13 secs, 285 ms, 480 μs, and 4 hnsecs
uint_stein: 9 secs, 173 ms, 414 μs, and 8 hnsecs
ulong_euclid: 20 secs, 227 ms, 886 μs, and 2 hnsecs
ulong_stein: 9 secs, 181 ms, 300 μs, and 8 hnsecs

[dlang-bot]

Thanks for your pull request, @ibuclaw!

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Testing this PR locally

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dub run digger -- build "master + phobos#7976"

[ibuclaw]

@thewilsonator - Reverted the document change, as Euclid's algorithm is still used in the BigInt overload (tested with CrippledInt).