I originally adopted this elegant X25519 implementation from Mike
Hamburg's STROBE protocol framework at https://strobe.sourceforge.io/
for various internal projects.

This version has standalone endianness and word-width detection, plus
minor cosmetic changes to match the normal code style of my projects,
but is otherwise identical to upstream. It is portable but can detect
and take advantage of 128-bit integer types where they are available.

Prototypes for available operations are in x25519.h. Code using them
must be linked against x25519.c.