bigint mac3: tweak thresholds between algorithms
It's not too rigorous, but thresholds 32 and 256 give me better results.
Before:
test multiply_0 ... bench: 87 ns/iter (+/- 0)
test multiply_1 ... bench: 11,926 ns/iter (+/- 19)
test multiply_2 ... bench: 772,178 ns/iter (+/- 3,068)
test multiply_3 ... bench: 2,034,237 ns/iter (+/- 9,618)
After:
test multiply_0 ... bench: 87 ns/iter (+/- 0)
test multiply_1 ... bench: 11,927 ns/iter (+/- 64)
test multiply_2 ... bench: 672,440 ns/iter (+/- 3,570)
test multiply_3 ... bench: 1,577,065 ns/iter (+/- 11,137)
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