* 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>

* `to_bits` -> `to_bits_le`
* `BitIterator` -> `BitIteratorLE` + `BitIteratorBE`
* `found_one`/`seen_one` -> `BitIteratorBE::without_leading_zeros`

This would be necessary if, in another crate, one wants to implement operators.

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