net/netip: reduce allocations in Addr.String for v4-in-v6 addresses

net/netip: reduce allocations in Addr.String for v4-in-v6 addresses

ip.Unmap will always return an Addr with ip.z == z4 in case of an
v4-in-v6 address. Thus, Addr.string4 can be called directly without the
additional indirection.

name                      old time/op    new time/op    delta
IPString/v6_v4-4             108ns ± 3%      74ns ± 4%  -31.23%  (p=0.000 n=9+10)
IPStringExpanded/v6_v4-4    89.6ns ± 6%    77.2ns ± 3%  -13.91%  (p=0.000 n=10+10)
AddrPortString/v6_v4-4       253ns ± 8%     197ns ± 3%  -22.13%  (p=0.000 n=10+10)

name                      old alloc/op   new alloc/op   delta
IPString/v6_v4-4             40.0B ± 0%     24.0B ± 0%  -40.00%  (p=0.000 n=10+10)
IPStringExpanded/v6_v4-4     48.0B ± 0%     48.0B ± 0%     ~     (all equal)
AddrPortString/v6_v4-4       77.0B ± 0%     61.0B ± 0%  -20.78%  (p=0.000 n=10+10)

name                      old allocs/op  new allocs/op  delta
IPString/v6_v4-4              2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)
IPStringExpanded/v6_v4-4      1.00 ± 0%      1.00 ± 0%     ~     (all equal)
AddrPortString/v6_v4-4        4.00 ± 0%      3.00 ± 0%  -25.00%  (p=0.000 n=10+10)

Change-Id: Id4affaf7a493aa11579c48721294f2e5889a8bef
Reviewed-on: https://go-review.googlesource.com/c/go/+/403914
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>