net: reduce allocation size in ReadFromUDP

Switch to concrete types. Bring your own object to fill in.

Allocate just enough for the IP byte slice.
The allocation is now just 4 bytes for IPv4,
which puts it in the tiny allocator, which is much faster.

name                  old time/op    new time/op    delta
WriteToReadFromUDP-8    13.7µs ± 1%    13.4µs ± 2%   -2.49%  (p=0.000 n=10+10)

name                  old alloc/op   new alloc/op   delta
WriteToReadFromUDP-8     32.0B ± 0%      4.0B ± 0%  -87.50%  (p=0.000 n=10+10)

name                  old allocs/op  new allocs/op  delta
WriteToReadFromUDP-8      1.00 ± 0%      1.00 ± 0%     ~     (all equal)

Windows is temporarily stubbed out.

Updates #43451

Change-Id: Ief506f891b401d28715d22dce6ebda037941924e
Reviewed-on: https://go-review.googlesource.com/c/go/+/331490


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Damien Neil <dneil@google.com>