bigint: small to_str_radix optimization
bigint: small to_str_radix optimization

Before:

test fac_to_string     ... bench:       1,630 ns/iter (+/- 34)
test fib_to_string     ... bench:         359 ns/iter (+/- 11)
test to_str_radix_02   ... bench:       3,097 ns/iter (+/- 19)
test to_str_radix_08   ... bench:       1,146 ns/iter (+/- 38)
test to_str_radix_10   ... bench:       4,248 ns/iter (+/- 36)
test to_str_radix_16   ... bench:         881 ns/iter (+/- 44)
test to_str_radix_36   ... bench:       8,073 ns/iter (+/- 75)

After:

test fac_to_string     ... bench:       1,492 ns/iter (+/- 20)
test fib_to_string     ... bench:         368 ns/iter (+/- 7)
test to_str_radix_02   ... bench:       2,038 ns/iter (+/- 47)
test to_str_radix_08   ... bench:         812 ns/iter (+/- 9)
test to_str_radix_10   ... bench:       3,919 ns/iter (+/- 40)
test to_str_radix_16   ... bench:         703 ns/iter (+/- 58)
test to_str_radix_36   ... bench:       7,852 ns/iter (+/- 81)

jbcrail/num,jbcrail/num,koverstreet/num,anchovieshat/num,koverstreet/num,rust-num/num-integer,rust-num/num,rust-num/num-integer,rust-num/num,vks/num,rust-num/num-traits,rust-num/num-bigint,rust-num/num-traits,vks/num,anchovieshat/num,DonSheddow/num,rust-lang/num,rust-num/num-bigint,rust-lang/num,DonSheddow/num