Breaking: changed Parity serialization to u8

[Kixunil]

Serializing the value as u8 is more compact but this is a breaking
change.

Visitor was renamed to avoid hungarian notation and maybe allow other
integers in the future.

For next major version, depends on #400

[tcharding]

tACK e6cb588

[Kixunil]

@tcharding your ACK got me to think about this again and maybe we should impl deserialize to suggest deserializing u8 but support all integers. That way it'd be backwards-compatible for typed formats. (Untyped binary formats are screwed but that's the case anyway 🤷‍♂️ )

[tcharding]

My initial thoughts are to not support all integers because there is no real limit to all so what ever subset we support is arbitrary. (I know this argument is a bit contrived.) Is there a use case you can think of where having support for additional integer sizes would be useful?

[Kixunil]

We would accept old formats. But really is there any harm accepting 0u64 and such? The only issue I can see is not being strict can make debugging serializers harder. I think it'd be fine at least for i32.

[tcharding]

I don't know if it matters but it would also mean deserialize -> seralize roundtrip will not produce the same output as input.

[Kixunil]

Doesn't seem important to me...

[apoelstra]

Is this ready for review?

[Kixunil]

Ready

[apoelstra]

ACK e6cb588