crypto.salsa20: make the number of rounds a comptime parameter

[jedisct1]

...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.