slh-dsa: no_std broken due to use of concat()

[erdnaxe]

When building a project for thumbv7em-none-eabihf target, cargo fails with the following error:

   Compiling slh-dsa v0.1.0
error[E0599]: no method named `concat` found for array `[&[u8]; 4]` in the current scope
   --> /home/redacted/.cargo/registry/src/index.crates.io-6f17d22bba15001f/slh-dsa-0.1.0/src/signing_key.rs:146:56
    |
146 |         let ctx_msg = [&[0], &ctx_len_bytes, ctx, msg].concat();
    |                                                        ^^^^^^ method not found in `[&[u8]; 4]`

error[E0599]: no method named `concat` found for array `[&[u8]; 4]` in the current scope
  --> /home/redacted/.cargo/registry/src/index.crates.io-6f17d22bba15001f/slh-dsa-0.1.0/src/verifying_key.rs:83:56
   |
83 |         let ctx_msg = [&[0], &ctx_len_bytes, ctx, msg].concat();
   |                                                        ^^^^^^ method not found in `[&[u8]; 4]`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `slh-dsa` (lib) due to 2 previous errors

It would be nice to build all crates in no_std mode in CI to detect such breakage.
Maybe we should add cargo check --no-default-features --target thumbv7em-none-eabihf in GitHub Actions to force no_std?

[tarcieri]

Fixed in #860