* 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)