Marshaler and Unmarshaler

[jeschkies]

I'd like to add marhsalling and unmarshalling to this package. I'd implement encoding.BinaryMarshaler and the smallBytes format from the Java reference implementation. Alternatively, I could add AsBytes and AsSmallBytes method. Is there any interest?

I could also go straight to Protobuf if there's interest. E.g. DDSketch Go comes with a Protobuf definition.