Co-authored-by: Pratyush Mishra <pratyushmishra@berkeley.edu>

* Implement ToConstraintFieldGadget for Vec<UInt8>

* Add documentation and implementation for slice

* Update documentation

* When a group element is a constant, precompute multiples of powers of two, and perform simple conditional additions (no doubling!).
* For short weierstrass curves, addition with a constant now uses mixed addition, which results in lower constraint weight.
* For short weierstrass curves, scalar multiplication now uses mixed addition, saving 1 constraint per bit of the scalar, along with lower constraint weight (at the cost of a small constant number of constraints to check for edge cases)

Co-authored-by: Pratyush Mishra <pratyushmishra@berkeley.edu>

* allow cs being none for into_edwards

This is related to a Zexe issue https://github.com/scipr-lab/zexe/issues/289

Basically, ToBitsGadget for FpVar may produce shorter output for a constant value, which is undesired for a few reasons.

The prior PR (#9) multiplied by `self.z` instead of `self.z.inverse()`.

Adds constraints to check that the conversion to affine coordinates happened correctly.

Co-authored-by: Nicholas Ward <npward@berkeley.edu>