hash2curve: configurable mapping function #1422
Description
If we want to eventually extend hash2curve to curve25519-dalek, it seems we might want to consider making the mapping function configurable. While I'm not aware of any alternative mapping functions for our curves in this repo, Edwards25519 has the alternative Lizard mapping and is in the works of being added to curve25519-dalek: dalek-cryptography/curve25519-dalek#826.
With #1389 the change should be quite straightforward: adding another generic to the standalone functions for MapToCurve. MapToCurve itself needs to be split up into a new trait just containing the parameters like SecurityLevel and just the mapping itself.