Only secp256k1_XMD:SHA-256_SSWU_RO_ and secp256k1_XMD:SHA-256_SSWU_RO_
are provided as high-level suites, and I make the explicit executive
decision not to expose expand_message_xmd.

Agility is footgun, and people that really need to do different suites
are free to use `Point.SetUniformBytes`.

The expand_message_xmd implementation and the test vector related
routines were swiped from a previous project.