Provide a way for every message to be encoded/decoded to/from NEO wire
encoding. For this introduce Msg interface with wire coding methods and
provide such methods for all message types.

For selected types the methods are implemented manually.
For most of the types the methods are generated automatically by protogen.go program.

protogen.go was mentioned in http://navytux.spb.ru/~kirr/neo.html#development-overview
in "On server-side NEO/go work started by first implementing messages
serialization in exactly the same wire format as NEO/py does ..." paragraph.

A bit of late protogen fixups history:

	lab.nexedi.com/kirr/neo/commit/c884bfd5
	lab.nexedi.com/kirr/neo/commit/385d813a
	lab.nexedi.com/kirr/neo/commit/0f7e0b00
	lab.nexedi.com/kirr/neo/commit/de3ef2c0

Also a message type can be reverse-looked up by message code via MsgType().
This will be later used in network receive code path.