The only reason we need `hex-conservative` is to parse strings and
format them as hex. For users that do not require this functionality we
can make the `hex-conservative` crate an optional dependency.
The `serde` feature requires `Display` so we enable `hex` from the
`serde` feature.
If `hex` feature is not enabled we still need to be able to debug so
provide `fmt::Debug` functionality by way of macros.
Close: #2654
ec06028f63