Edwards Curves X25519 and X448 from RFC 7748

This library implements the ECDH functions 'X25519' and 'X448' as specified in RFC 7748, 'Elliptic curves for security'. In the spirit of the original publications, the public API is kept as simple as possible to make it easy to use and hard to misuse.

The current version is written in plain OCaml, leveraging Zarith for integer arithmetic. While this makes the implementation straightforward and easy to reason about, the performance is nowhere near that of DJB's implementation using floating point registers (https://cr.yp.to/ecdh.html).