replaced custom_derive with macro-attr

[alexanderkjall]

as macro-attr supersedes custom_derive.

ref #13

[jayvdb]

ping @DanielKeep

https://github.com/trailofbits/cargo-unmaintained lists custom_derive as unmaintained

custom_derive (not in https://github.com/DanielKeep/rust-custom-derive)

[jayvdb]

I've created rustsec/advisory-db#2199