Unimplemented hash to curve

[GhostOfGauss]

Currently the halo2 CurveExt trait is not fully implemented except for the pasta curves. The missing function hash_to_curve is unimplemented (see here). The implementation for pasta curves (here) makes use of an algorithm valid only for curves with a*b = 0 (see here). This implementation should therefore work for bn256 and secp256k1, which both have a = 0.

My first attempt to copy-paste the pasta implementation over gets stuck on the alloc dependency. Once that's figured out I'd be happy to open a PR

[eldenpark]

I'd also like to know how this is going to be handled. Have you made any progress w/ that alloc feature? @GhostOfGauss

[han0110]

Replaced by #70