bigint: slightly improve multiply/divide performance
Using `vec![0; len]` initialization is a little faster.
Before:
test multiply_0 ... bench: 354 ns/iter (+/- 6)
test multiply_1 ... bench: 33,966 ns/iter (+/- 1,508)
test multiply_2 ... bench: 3,663,686 ns/iter (+/- 60,880)
test divide_0 ... bench: 891 ns/iter (+/- 51)
test divide_1 ... bench: 17,316 ns/iter (+/- 387)
test divide_2 ... bench: 1,290,378 ns/iter (+/- 73,016)
After:
test multiply_0 ... bench: 351 ns/iter (+/- 39)
test multiply_1 ... bench: 30,827 ns/iter (+/- 680)
test multiply_2 ... bench: 3,692,968 ns/iter (+/- 91,146)
test divide_0 ... bench: 902 ns/iter (+/- 14)
test divide_1 ... bench: 16,981 ns/iter (+/- 102)
test divide_2 ... bench: 1,146,367 ns/iter (+/- 60,152)