Implement echo server
o Default state alphabet=None prevents testing input symbol type
o Implement transition greedy=True forces consuming all input symbols
  possible, even if terminal state reached (false terminates ASAP)
o Building a state machine from greenery.fsm allows encoder to convert
  greenery.fsm symbols to process input symbols in a different alphabet
  (eg. fsm in utf-8, input in bytes).  Any states added also duplicate
  default transition.
o Test greenery.fsm and cpppo handling of FMSs specified in UTF-8
  symbols, transformation of resultant machine by encoding to bytes
o dotdict can now delete entries; values, and empty dotdict key layers
o misc.natural key sorting function supports None entries
o cpppo.server.echo handles multiple threads, large volumes of data,
  and tests to ensure all threads get all their data back correctly.