Implement Wei25519 to exercise some more elliptic curve code

Implement Wei25519 to exercise some more elliptic curve code

This provides a non-trivial non-builtin curve that is unlikely to ever
become a builtin curve. This exercises the cofactor guessing code and
and ensures that things work as far as they can with a custom OID. The
main reason for adding it is to have a "real-world" example for an
upcoming check that EC parameters represent a builtin curve.