Implement map data type.

[redstar]

The map data type has a special proto syntax.
The serialization of

map<key_type, value_type> map_field = N;

is backward compatible to

message MapFieldEntry {
  key_type key = 1;
  value_type value = 2;
}

repeated MapFieldEntry map_field = N;

The option [packed = true] is not allowed on map fields.

[redstar]

I still work on the serialization. My question is if the general design is welcomed or if there are better ideas.

[msoucy]

I have been away from dlang for a while (basically since graduating college), but I took a look at your design and it seems like a good starting point. If you're still open to developing this, I can take a look once serialization is implemented.