This patch adds proto.Encoding('M') and teaches it to encode/decode
messages via MessagePack by the rules defined in

	nexedi/neoppod@9d0bf97a
	( nexedi/neoppod!11 )

It only adds support for messages serialization, without changing
proto.go to match changes in e.g. enums reordering, and without adding
support for MessagePack at link layer in neonet.

M-encoding was only tested for NEO/go-NEO/go, and was not yet tested for
NEO/go-NEO/py interoperation. There will be likely small mistakes
discovered on my side that should be hopefully easy to fix step by step
once we get to that phase.