MBEDTLS_CHACHA20_H macro

\file chacha20.h This file contains ChaCha20 definitions and functions. ChaCha20 is a stream cipher that can encrypt and decrypt information. ChaCha was created by Daniel Bernstein as a variant of its Salsa cipher https://cr.yp.to/chacha/chacha-20080128.pdf ChaCha20 is the variant with 20 rounds, that was also standardized in RFC 7539. \author Daniel King <damaki.gh@gmail.com>