Consider using group's Wnaf/WnafGroup

[kayabaNerve]

group defines a struct for tables already. WnafGroup provides recommended window parameters, which would be able to replace multiexp's current amalgamation of dalek and k256 performance.

It may be more beneficial to roll our own solution, as this would have implications for #41 and #42, and will likely be optimal to write our own code to solve those first BEFORE considering Wnaf.

WnafGroup alone would be very beneficial for multiexp, yet it isn't widely supported. I've opened an issue for k256 and p256 however, and we can provide support for dalek.

May have a relation to zkcrypto/group issues/25.

EDIT: I explicitly removed github.com from the last item there, trying not to ping zkcrypto's issue as it wasn't a relevant item to them at this time, IMO. Turns out GH still figured it out. Now I know GH does that.