* master: (25 commits)
  go/neo/neonet: Teach Dial to autodetect server encoding
  go/neo/neonet: MessagePack support for link layer (draft)
  go/neo/proto: Cosmetics
  go/neo/proto: MessagePack support for messages (draft)
  go/neo/internal/msgpack: New package with runtime support for MessagePack
  go/neo/proto: protogen: Prepare to generate code for struct heads
  go/neo/proto: protogen: Factor-out handling of slice and map heads into common place
  go/neo/proto: protogen: Fix naming thinko in decoderN.genBuf
  go/neo/proto: Introduce Encoding
  go/neo/proto: Unexport Msg.NEOMsg{Encode,Decode}
  go/neo/proto: Kill outdated TODO
  go/neo/proto: Unexport Msg.NEOMsgCode
  go/neo/proto: protogen: Reuse sizer for typeSizeFixed
  go/neo/proto: protogen: Load zodb.Tid and zodb.Oid types
  go/neo/proto: protogen: Factor-out code to lookup object for <pkg>.<name> into common function
  go/neo/proto: protogen: Clarify criteria to detect []byte
  go/neo/proto: protogen: Fix it not to silently skip a pointer field
  go/neo/proto: AddrString: Also treat lonet networks as having addr in host:port form
  go/neo/neonet: Adjust pktAlloc to always allocate pktBuf with cap >= PktHeaderSize
  go/neo/neonet: Factor-out decode of packet header into pktHeadDecode
  ...

NEO/go server accepts preferred encoding of client from start, but
NEO/py server implements either N or M encoding and disconnects client
silently if handshake is not exactly what is expected.

However we can teach Dial to retry with different preferred options and
this way when connection to a NEO/py server, it will essentially
autodetect which encoding is used and accepted by remote peer.

This patch adds support for serializing packet frames with M encoding on
the wire. To do so it follows rules defined in

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

Server handshake is reworked to autodetect client's preferred encoding.
Client always prefers 'N' for now.

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.

It complements github.com/tinylib/msgp with things that neo/proto needs
to encode/decode messages.

It is noop for 'N' encoding, but will be non-empty for MessagePack.