bigint: improve from_str_radix performance
Before:
test from_str_radix_02 ... bench: 8,432 ns/iter (+/- 280)
test from_str_radix_08 ... bench: 7,397 ns/iter (+/- 95)
test from_str_radix_10 ... bench: 7,344 ns/iter (+/- 142)
test from_str_radix_16 ... bench: 6,753 ns/iter (+/- 157)
test from_str_radix_36 ... bench: 7,093 ns/iter (+/- 60)
After:
test from_str_radix_02 ... bench: 3,295 ns/iter (+/- 81)
test from_str_radix_08 ... bench: 1,377 ns/iter (+/- 56)
test from_str_radix_10 ... bench: 1,583 ns/iter (+/- 16)
test from_str_radix_16 ... bench: 1,483 ns/iter (+/- 53)
test from_str_radix_36 ... bench: 1,628 ns/iter (+/- 27)