net: remove allocation from UDPConn.WriteTo

net: remove allocation from UDPConn.WriteTo

Duplicate some code to avoid an interface.

name                  old time/op    new time/op    delta
WriteToReadFromUDP-8    6.38µs ±20%    5.59µs ±10%  -12.38%  (p=0.001 n=10+9)

name                  old alloc/op   new alloc/op   delta
WriteToReadFromUDP-8     64.0B ± 0%     32.0B ± 0%  -50.00%  (p=0.000 n=10+10)

name                  old allocs/op  new allocs/op  delta
WriteToReadFromUDP-8      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)

Windows is temporarily stubbed out.

Updates #43451

Change-Id: Ied15ff92268c652cf445836e0446025eaeb60cc9
Reviewed-on: https://go-review.googlesource.com/c/go/+/331489
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>