ChaCha20 implementation for encryption

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

rtlplayground.c

@@ -28,6 +28,7 @@
 #include "httpd/page_impl.h"
 #include "boot.h"
 #include "sfp.h"
+#include "crypto/chacha.h"
 
 extern __code const struct machine machine;
 extern __xdata uint32_t flash_size;