Draft: Support many-to-many mapping between relay cells and messages

• Add RelayCellAccumulator to transform a stream of cells to a stream of
  messages.
• Add RelayMsgEncoder to transform a relay message to a stream of cells
  (and optionally pack extra data into empty space in the last cell).
• Remove and modify some previous encoding and decoding APIs to force
  encoding and decoding to go through above objects.
• Update the circuit reactor to use the new APIs.

XXX: arti client compiles, but tests don't yet.
XXX: need to update semver.md