...instead of hard-coding it to 20.

- This is consistent with the ChaCha implementation
- NaCl and libsodium, that this API is designed to interop with,
also support 8 and 12 round variants. The 12 round variant, in
particular, provides the same security level as the 20 round variant,
but is obviously faster.
- scrypt currently uses its own non optimized version of Salsa, just
because it use 8 rounds instead of 20. This will help remove code
duplication.

No behavior nor public API changes. The Salsa20 and XSalsa20 still
represent the 20-round variant.