Add an AppendPack method to Message that appends the message data into a
byte buffer. Reusing a buffer allows for a reduction in allocations.

name          time/op
Pack-8        5.04µs ± 1%
AppendPack-8  4.95µs ± 2%

name          alloc/op
Pack-8        6.22kB ± 0%
AppendPack-8  5.71kB ± 0%

name          allocs/op
Pack-8          21.0 ± 0%
AppendPack-8    20.0 ± 0%

Change-Id: I8bb6b07787cf2ba9ef32e1e60a3003a585ec55be
Reviewed-on: https://go-review.googlesource.com/45274Reviewed-by: Ian Gudger <igudger@google.com>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>