slh-dsa: fix multiple AsBytes definitions

[erdnaxe]

For some reasons, I got the following build error while using slh-dsa in a no_std project:

[...]
error[E0252]: the name `AsBytes` is defined multiple times
  --> /home/user/.cargo/git/checkouts/signatures-f73d5010bc56a463/cfda25f/slh-dsa/src/address.rs:23:5
   |
22 | use zerocopy::AsBytes;
   |     ----------------- previous import of the macro `AsBytes` here
23 | use zerocopy_derive::AsBytes;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^ `AsBytes` reimported here
   |
   = note: `AsBytes` must be defined only once in the macro namespace of this module

For more information about this error, try `rustc --explain E0252`.
error: could not compile `slh-dsa` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...

I don't really understand why cargo test in RustCrypto/signatures/slh-dsa works fine. Here is a proposition for a fix, but maybe there is a cleaner way to address this issue ?

[tarcieri]

It seems like the problem here is that zerocopy conditionally re-exports zerocopy_derive::AsBytes when its derive feature is enabled, but slh-dsa is not enabling it.

What probably happened @erdnaxe is some other crate in your project enabled the zerocopy/derive feature, triggering the re-export.

If you explicitly enable zerocopy's derive feature, then it should re-export the zerocopy::AsBytes derive macro, making the direct dependency and imports from zerocopy-derive unnecessary.

[tarcieri]

I think #847 should take care of it

[erdnaxe]

Thank you! I confirm that it fixes the build error.