Protobuf reader/writer modularisation

Modularises the core protobufIO (de)serialize functions.

This allows other code to use the GenEvent/GenRunInfo -> protobuf byte string functions for other uses. Specifically looking at writing an event server that sends events over the wire. For many cases we can just use the full Reader/Writer implementations to write to a binary stream that is then sent over the transport layer, but this separate gives a little more flexibility of implementation.

Slows things down very slightly due to an extra copy or two, but speed is not yet the focus of this IO module. There will be options to speed things up that I will investigate in the future.