Small enchancement to the core functionality of ChaCha20 stream cipher

[blackshirt]

This pull request adds some minor enhancement to the xor_key_stream and other core functionality, especially in speed of execution.
Local bench on my low end laptop shows up small nice improvement, like i paste on discord
Before patched

$ v run bench.v
 SPENT  7206.581 ms in chacha20.encrypt 40 bytes of plaintext with 500000 iterations, total bytes encrypted : 20000000 bytes
 SPENT  8056.834 ms in chacha20.encrypt 64 bytes of plaintext with 500000 iterations, total bytes encrypted : 32000000 bytes
 SPENT 11103.743 ms in chacha20.encrypt 128 bytes of plaintext with 500000 iterations, total bytes encrypted : 64000000 bytes
 SPENT 22957.365 ms in chacha20.encrypt 200 bytes of plaintext with 500000 iterations, total bytes encrypted : 100000000 bytes

And after being patched

$ v run bench.v
 SPENT  5193.056 ms in chacha20.encrypt 40 bytes of plaintext with 500000 iterations, total bytes encrypted : 20000000 bytes
 SPENT  5925.972 ms in chacha20.encrypt 64 bytes of plaintext with 500000 iterations, total bytes encrypted : 32000000 bytes
 SPENT 11119.856 ms in chacha20.encrypt 128 bytes of plaintext with 500000 iterations, total bytes encrypted : 64000000 bytes
 SPENT 19557.270 ms in chacha20.encrypt 200 bytes of plaintext with 500000 iterations, total bytes encrypted : 100000000 bytes

This PR adds some changes by :

• Adds @[direct_array_access] attributes to the core function that likely needed
• Reduces allocation of output buffer by using dst buffer directly
• Use builtin vmemset to reset cipher's key and nonce
• Some other minor changes

Please gives a review, comment and feedback.
Best regards

[spytheman]

Excellent work.