Centralize handling of strings and bytes arrays for event formatters

[jskeet]

All event formatters are expected to handle strings and byte arrays. We should make it really easy for them to do so via common code, e.g. protected methods in CloudEventFormatter.

[jskeet]

This hasn't been done for 2.0; I still have the Protobuf event formatter to finish though, so when that's done it will give us more data.

[bencyoung-Fignum]

Would this potentially let us inject things like https://github.com/microsoft/Microsoft.IO.RecyclableMemoryStream/blob/master/src/RecyclableMemoryStream.cs into the serialization? and remove the use of Encoding.GetBytes and Encoding.GetString etc? It's hard for me to see how I could do this without cloning the current class entirely

[jskeet]

@bencyoung-Fignum: Honestly couldn't say yet - but maybe.