Consider dropping `ByteString` type from public API

[bnewbold]

I think the use of this type can be confusing to users. We should instead use AsRef in public method arguments, and simply return Vec<u8> or String (as appropriate) in return types.

We could continue using ByteString internally, if it is helpful.

Filing this issue as a placeholder and for discussion, this is not urgent, and i'm not planning to do it in the near future.