[crypto] Add X25519 key exchange algorithm

[crypto] Add X25519 key exchange algorithm

Add an implementation of the X25519 key exchange algorithm as defined
in RFC7748.

This implementation is inspired by and partially based upon the paper
"Implementing Curve25519/X25519: A Tutorial on Elliptic Curve
Cryptography" by Martin Kleppmann, available for download from
https://www.cl.cam.ac.uk/teaching/2122/Crypto/curve25519.pdf

The underlying modular addition, subtraction, and multiplication
operations are completely redesigned for substantially improved
efficiency compared to the TweetNaCl implementation studied in that
paper (approximately 5x-10x faster and with 70% less memory usage).

Signed-off-by: Michael Brown <mcb30@ipxe.org>