Optimizations for multiplication

[cuviper]

Relevant benchmarks:

 factorial_100          5,137                 2,663                      -2,474  -48.16%   x 1.93
 factorial_div_biguint  791,253               773,134                   -18,119   -2.29%   x 1.02
 factorial_div_u32      766,341               748,398                   -17,943   -2.34%   x 1.02
 factorial_mul_biguint  113,269               58,480                    -54,789  -48.37%   x 1.94
 factorial_mul_u32      35,062                37,848                      2,786    7.95%   x 0.93
 multiply_0             39                    40                              1    2.56%   x 0.98
 multiply_1             3,693                 3,708                          15    0.41%   x 1.00
 multiply_2             309,911               215,371                   -94,540  -30.51%   x 1.44
 multiply_3             662,091               494,419                  -167,672  -25.32%   x 1.34
 pow_bench              2,550,050             2,144,069                -405,981  -15.92%   x 1.19
 pow_bench_1e1000       1,332                 1,071                        -261  -19.59%   x 1.24
 pow_bench_1e10000      62,234                36,109                    -26,125  -41.98%   x 1.72
 pow_bench_1e100000     2,530,757             1,134,613              -1,396,144  -55.17%   x 2.23
 pow_bench_bigexp       2,630,660             2,155,688                -474,972  -18.06%   x 1.22

[cuviper]

bors r+

[bors]

Build succeeded:

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• Test (1.31.0)
• Test (1.34.0)
• Test (1.36.0)
• Test (1.40.0)
• Test (1.46.0)
• Test (beta)
• Test (i686)
• Test (nightly)
• Test (stable)

[renatoathaydes]

Hi! I've run a little benchmark comparing the performance of some code I have that uses BigUint using version 0.4.0 and then the master branch of this repo, which already has this fix...

Here's how much difference it makes:

As this was merged on the 14th of March, I assume it has not been released yet as the last release was on the 05th of March?

When will this be released, it really improves performance by a lot!

Thanks!

[cuviper]

Thanks for the nudge -- I've now published 0.4.1!