salsa20: do not use zeroize default features to prevent alloc inclusion

[Wassasin]

Should be OK as this crate does not use any zeroize alloc functionality.

Stumbled on this when developing embedded firmware, and got the following message during build of the binary:

error: no global memory allocator found but one is required; link to std or add `#[global_allocator]` to a static item that implements the GlobalAlloc trait.

[newpavlov]

Thank you! Can you please also do the same for hc-256 and rabbit crates?

@tarcieri
We probably should test no_std builds in CI with enabled zeroize feature as well.

[tarcieri]

@newpavlov yeah, in many of the crates the build job tests various feature combinations, but this one appears to have been overlooked

[tarcieri]

I'll take care of hc-256 and rabbit