Move Erlang code into separate library

[oubiwann]

See geomyidia/erlcmd#2

Let's do some cleanup before this happens, though:

Tasks:

• Cleanup work (make the erl package easily separable)
  • Re-do erl datatypes and data Erlang <-> Go #35
  • Fix up existing abstractions, separating erlang_go types and midiserver types
  • Add missing tests
  • Consolidate work around packet-parsing, binary data, and type conversions
  • Update to the latest erlang_go #32
  • Convert current use of okeuday/erlang_go to ergo/etf #33
• Move erl subpackage to the erlcmd repo
• Update midiserver to use the erlcmd package(s)
  • Delete code that's now in erlcmd
  • Be sure to keep the following:
    • erl/server
    • erl/rpc
    • erl/midi
    • erl/epmd
    • erl/erl.go (but move the code somewhere appropriate)
• Overhaul MIDI-handling code #37
• Update CLI command handling #36
• Cleanup message abstraction / responsibilities