Client-side connection objects must now provide both client and server
endpoint types. When a message is received from the server side, we try
to decode it using both endpoint types and then send it to the right
place for handling.
This now makes it possible for AudioServer to send unsolicited messages
to its clients. This opens up a ton of possibilities :^)
This was a workaround to be able to build on case-insensitive file
systems where it might get confused about <string.h> vs <String.h>.
Let's just not support building that way, so String.h can have an
objectively nicer name. :^)
- Add IEndpoint::handle(IMessage), a big switch table on message type.
handle() will return a response message for synchronous messages,
and return nullptr otherwise.
- Use i32 instead of int for everything
- Make IMessage::encode() const
- Make IEndpoint::decode_message() static, this allows template code to
decode messages without an endpoint instance on hand.
