Decoding should return how much of the buffer was used

[reem]

This is so it is possible to parse from a large (e.g. ring) buffer and know when the current thing we are decoding has ended and another message has begun.

[TyOverby]

Good idea; I actually already collect that information, I'll just need to export it.

[TyOverby]

I'm reverting this because I don't think that the information is useful enough to put on the basic functions.

You can still access this information by constructing the DecoderReader, decoding from that, then asking how many bytes were read.