The current division algorithm produces one word of result at a time,
using 2-word division to compute the top word and mulAddVWW to compute
the remainder. The top word may need to be adjusted by 1 or 2 units.

The recursive version, based on Burnikel, Ziegler, "Fast Recursive Division",
uses the same principles, but in a multi-word setting, so that
multiplication benefits from the Karatsuba algorithm (and possibly later
improvements).

benchmark                             old ns/op        new ns/op      delta
BenchmarkDiv/20/10-4                  38.2             38.3           +0.26%
BenchmarkDiv/40/20-4                  38.7             38.5           -0.52%
BenchmarkDiv/100/50-4                 62.5             62.6           +0.16%
BenchmarkDiv/200/100-4                238              259            +8.82%
BenchmarkDiv/400/200-4                311              338            +8.68%
BenchmarkDiv/1000/500-4               604              649            +7.45%
BenchmarkDiv/2000/1000-4              1214             1278           +5.27%
BenchmarkDiv/20000/10000-4            38279            36510          -4.62%
BenchmarkDiv/200000/100000-4          3022057          1359615        -55.01%
BenchmarkDiv/2000000/1000000-4        310827664        54012939       -82.62%
BenchmarkDiv/20000000/10000000-4      33272829421      1965401359     -94.09%
BenchmarkString/10/Base10-4           158              156            -1.27%
BenchmarkString/100/Base10-4          797              792            -0.63%
BenchmarkString/1000/Base10-4         3677             3814           +3.73%
BenchmarkString/10000/Base10-4        16633            17116          +2.90%
BenchmarkString/100000/Base10-4       5779029          1793808        -68.96%
BenchmarkString/1000000/Base10-4      889840820        85524031       -90.39%
BenchmarkString/10000000/Base10-4     134338236860     4935657026     -96.33%

Fixes #21960
Updates #30943

Change-Id: I134c6f81a47870c688ca95b6081eb9211def15a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/172018Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>