Update the built-in Chacha20-Poly1305 code to an RFC 7539 complaint version.

Update the built-in Chacha20-Poly1305 code to an RFC 7539 complaint version.

This is necessary so our copy of Chacha20-Poly1305 is compatible with that
of many other crypto libraries.

This code is made by Grigori Goronz, but is heavily based on the code from
D.J. Bernstein's ref10 implementation used before.