Erlang: nextgen generator + adapted runtime library

Key highlights:

 * (!) respect namespaces
 * improved, more detailed typespec generation
 * (!) maps are now, well, maps (Erlang > 17.0 required)
 * sets -> ordsets
 * enum_info to reflect upon thrift enumerations
 * (!) enum fields come out of the wire as atoms now (better to make it into an option)
 * fixed indentation