Import implementations of X25519 and X448 from Catacomb.

Taken from https://git.distorted.org.uk/~mdw/catacomb/, commit-id
9c1437f372e62f0b3b3a7162aabee73bdc96ce4b.

These are just the raw files from upstream.  Don't try to build them,
because they have all sorts of dependencies on the rest of the library.
But at least this way I can record the hacking I did to integrate them
into Secnet.

The test vector files were originally called things like
`math/t/f25519': I've renamed them here because otherwise they'd briefly
introduce directories or have very unhelpful names.  They'll need
radically reformatting anyway, because the test-vector parser didn't
come along for the ride.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>