Add Encodable/Decodable protocols

[ulissesalmeida]

This feature adds the ability to convert to and from protobufs structs and elixir terms.

The ideas are from this issue: #89

Usage:

If you want to transform an automatically a protobuf message to an Elixir term after decoding, you can implement to_elixir function of the Protobuf.Decodable

      defimpl Protobuf.Decodable, for: MyApp.Protobuf.Date do
        def to_elixir(%MyApp.Protobuf.Date{year: year, month: month, day: day}) do
          {:ok, date} = Date.new(year, month, day)
          date
        end
      end
      # later in a decoded message
      proto_message.birthday
      ~D[1988-10-29]

If you want to automatically encode a elixir term to a protobuf message, you can implement the Encodable protocol.

      defimpl Protobuf.Encodable, for: Date do
        def to_protobuf(%Date{year: year, month: month, day: day}, MyApp.Protobuf.Date) do
          MyApp.Protobuf.Date.new(year: year, month: month, day: day)
        end
      end
      # later, you can use Elixir terms in your fields and those will be
      # converted to protobuf structs before binary encoding
      %{protobuf_message | birthday: ~D[1988-10-29]}

Then, if you do this, basically you need to annotate your types differently, then you can annotate your protobuf message with the Elixir message:

       message Date {
         option (elixirpb.message).typespec = "Date.t";
         int iso_days = 1;
       }

[ulissesalmeida]

Very weak test, but I don't know how is the best way to test generation from protobuf file to elixir file.

[whatyouhide]

Hey @ulissesalmeida, we decided that we can't take any new code in this library for now since we're trying to get it back in shape and releasing it. We want to keep only the features that we have right now without adding anything for the time being. We'll consider something like this before 1.0 though! Thank you so much 💟