Update the message handling from Erlang/LFE

[oubiwann]

The LFE examples project (which pulls this repo in) has just changed from native port to exec:

• Switch from portserver to execserver lfeutre/port-examples#3

As such, there are some messaging changes that will be necessary ... a good place to start is the Go midiserver, which writes messages back to an LFE exec server:

• https://github.com/ut-proj/midiserver/tree/release/0.2.x/pkg/erl

Although ... it seems like now might be the time to do this:

• Pull in code from midiserver erlcmd#2

In which case, once the code is moved there, that will be the place to look for how to update the echo example code to process messages from an Erlang exec server.