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.
