Make the usage of pairings generic in ZeroMorph implementation

[emmorais]

For now we are just using Grumpkin, but we should make our implementation generic to work over any elliptic curve that provides an appropriate bilinear pairing.

[huitseeker]

Helped by #28 (that is the code there is generic), but lurk-lab/arecibo#21 is needed to convert the current hiding variant to a generic form.

[huitseeker]

This is done in https://github.com/lurk-lab/arecibo/tree/zeromorph, please reopen if there's anything left