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