Add an implementation in 8051 assembler for the generation of a
ChaCha20 keystream and an encryption function using the keystream
in C.
A test function using the example from RFC7539 is provided in
chacha20_test.
Keys, nonce, and block counter and the pointers to the input plaintext,
its length and the output pointer are provided via a struct chacha20,
which also holds the working state of the ChaCha20 algorithm
