Refactor to protobuf

[tolauwae]

Refactors the existing debugger API using a protobuf scheme. The new scheme his optimized for low message size.

protobuf scheme

Protobuf defines two message type enums:

• Command: Frontend -> WARDuino
• Notification: WARDuino -> frontend

For instance:

enum Command {
  COMMAND_RUN = 0;                 // no payload
  COMMAND_HALT = 1;                // no payload
  //...
}

Payloads are defined in protobuf as messages. For instance:

message CodeLocation {
    uint32 module_index = 1;
    uint32 program_counter = 2;
}

message Breakpoint {
    CodeLocation location = 1;
}

Debug messages are not fully encoded by protobuf, only these payload.
This means that debug messages are of this form:

[type: uint8][payload length: varint][protobuf payload]

This allows for minimal message size. Sending message looks like this:

write_byte(COMMAND_CONTINUE_FOR);
write_length(payload_size);
pb_encode(&stream, ContinueFor_fields, &message);

Which results in this message on the wire:

┌────────────┬────────────┬───────────────────────────────┐
│ type       │ length     │ encoded ContinueFor (payload) │
│ 22         │ 2          │ 08 05                         │
└────────────┴────────────┴───────────────────────────────┘

ABI improvements

• adapted for multiple modules (vm support since Support for multiple modules #338)