This is optimization is only for IPv4. It allocates a result buffer and
writes the IPv4 octets as dotted decimal into it before converting
it to a string just once, reducing allocations.
Benchmark shows performance improvement:
name old time/op new time/op delta
IPString/IPv4-8 284ns ± 4% 144ns ± 6% -49.35% (p=0.000 n=19+17)
IPString/IPv6-8 1.34µs ± 5% 1.14µs ± 5% -14.37% (p=0.000 n=19+20)
name old alloc/op new alloc/op delta
IPString/IPv4-8 24.0B ± 0% 16.0B ± 0% -33.33% (p=0.000 n=20+20)
IPString/IPv6-8 232B ± 0% 224B ± 0% -3.45% (p=0.000 n=20+20)
name old allocs/op new allocs/op delta
IPString/IPv4-8 3.00 ± 0% 2.00 ± 0% -33.33% (p=0.000 n=20+20)
IPString/IPv6-8 12.0 ± 0% 11.0 ± 0% -8.33% (p=0.000 n=20+20)