Re-add SSE2 with soft fallback for non salsa20/20 variants

[oxarbitrage]

Here i re-added the SSE2 with the caveat of variants.

• Reverts https://github.com/RustCrypto/stream-ciphers/pull/346 - https://github.com/RustCrypto/stream-ciphers/commit/be4cdb3c965db975e12f568a0c76df3cc4dfefa4
• Add regression tests created in https://github.com/RustCrypto/stream-ciphers/pull/346 - https://github.com/RustCrypto/stream-ciphers/commit/f0781db2ab566d9fad9e11bb8f4b3c7df969e843
• Add the soft fallback for non salsa20/20 variants - https://github.com/RustCrypto/stream-ciphers/commit/3716c623121589669eba101c3be28940fef70504
• Add note about the change - https://github.com/RustCrypto/stream-ciphers/commit/8233bdac709f61e9921fc912241d1d7db4350eaf

[oxarbitrage]

@baloo i think your comments were good and i wanted to give it a try however i was not able to find the time yet, iirc i tried that before i was not able to make the whole thing work so i ended up with the current layout. i might want to give it a new look. can you readd the comments ?

[baloo]

@oxarbitrage I deleted my comments because they were incorrect. Situation is more complex than I initially thought.

[oxarbitrage]

@oxarbitrage I deleted my comments because they were incorrect. Situation is more complex than I initially thought.

That makes sense, as i think i already tried what was in those comments without being able to make it work. Thanks.

[baloo]

https://github.com/objectx/salsa20/blob/20d595b37597320a73acf87b3d3cc3ad3c9b66e6/src/salsa20.cxx#L182-L185 This is another implementation of salsa20 with sse2.

I like a lot the visual representation of the matrices in this one.

I still think it should be possible to reproduce the quarter round in the soft backend. I might try that at some point when I have more time.