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

* We add a double_and_add method that computes 2 * self + other more
  efficiently than just doubling + addition; this is not used anywhere 
  yet, but I am planning on fiddling with it to see if we can leverage
  it somehow. (See zcash/zcash#3924 for details)

* We handle constant scalars better:
  * We skip the most-significant constant zeroes to avoid unnecessary
    doubling
  * When intermediate bits of the scalar are constants, instead of
    conditionally adding, we directly use the value of the bit to
    decide whether to add or not.


Co-authored-by: Dev Ojha <ValarDragon@users.noreply.github.com>
Co-authored-by: weikeng <w.k@berkeley.edu>

Co-authored-by: Dev Ojha <ValarDragon@users.noreply.github.com>
Co-authored-by: Pratyush Mishra <pratyushmishra@berkeley.edu>

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

* Add changelog and .github issue templates

* 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>

* `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.