Rename hex_impl! to fmt_impl! and reuse it for fmt::Debug

[LogvinovLeon]

In fmt module there are two submodules:

• debug - Implementing fmt::Debug
• hex - Implementing fmt::LowerHex, fmt::UpperHex

Both implement core logic for BytesRef<'_>, but later one used a macro to propagate it to Bytes and BytesMut while other did it manually. This PR fixes that.

Macro imports in rust edition 2018 are limited. Therefore - I needed to first define a macro in a super module and later add #[macro_use] attribute to submodules.

I've tested that this solution does not leak the macro to the library public interface or docs.

Saves 9 lines of code ;) 16 insertions(+), 25 deletions(-)

[LogvinovLeon]

@Darksonn are there any other changes you want me to make to this PR?

[Darksonn]

Thanks.