net/netip: optimize AddrPort.String for IPv6 addresses

net/netip: optimize AddrPort.String for IPv6 addresses

Inline the IPv6-case of joinHostPort and avoid the check for a colon in
the address. The address is already known to be an IPv6 address. Also
use iota.Uitoa to convert the uin16 port.

name                          old time/op    new time/op    delta
AddrPortString/v4-8             45.7ns ± 0%    46.6ns ± 1%   +1.97%  (p=0.000 n=10+10)
AddrPortString/v6-8              106ns ± 3%      95ns ± 0%  -10.79%  (p=0.000 n=9+10)
AddrPortString/v6_ellipsis-8     110ns ± 0%      96ns ± 0%  -12.37%  (p=0.000 n=9+10)
AddrPortString/v6_v4-8          85.1ns ± 0%    70.8ns ± 0%  -16.80%  (p=0.000 n=9+10)
AddrPortString/v6_zone-8         110ns ± 0%      97ns ± 0%  -12.29%  (p=0.000 n=10+10)

name                          old alloc/op   new alloc/op   delta
AddrPortString/v4-8              24.0B ± 0%     24.0B ± 0%     ~     (all equal)
AddrPortString/v6-8               101B ± 0%       96B ± 0%   -4.95%  (p=0.000 n=10+10)
AddrPortString/v6_ellipsis-8     61.0B ± 0%     56.0B ± 0%   -8.20%  (p=0.000 n=10+10)
AddrPortString/v6_v4-8           61.0B ± 0%     56.0B ± 0%   -8.20%  (p=0.000 n=10+10)
AddrPortString/v6_zone-8         61.0B ± 0%     56.0B ± 0%   -8.20%  (p=0.000 n=10+10)

name                          old allocs/op  new allocs/op  delta
AddrPortString/v4-8               1.00 ± 0%      1.00 ± 0%     ~     (all equal)
AddrPortString/v6-8               3.00 ± 0%      2.00 ± 0%  -33.33%  (p=0.000 n=10+10)
AddrPortString/v6_ellipsis-8      3.00 ± 0%      2.00 ± 0%  -33.33%  (p=0.000 n=10+10)
AddrPortString/v6_v4-8            3.00 ± 0%      2.00 ± 0%  -33.33%  (p=0.000 n=10+10)
AddrPortString/v6_zone-8          3.00 ± 0%      2.00 ± 0%  -33.33%  (p=0.000 n=10+10)

Change-Id: I96a7f01c4e33bfa157a98558c62e2e0a8147726d
Reviewed-on: https://go-review.googlesource.com/c/go/+/542395
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>