Introduce msg.PackBuffer() - it's like msg.Pack() but can reuse a byte buffer

msg.Pack() always allocates a byte slice. This is good for simplicity,
but in a serious application it's preferable to reuse byte slices to
reduce the GC overhead. This patch introduces a new public method:
PackBuffer(). It's exaclty like Pack() but is able to reuse a
given byte slice. It will still allocate a new slice if the given one
is too small.