Audit fixes/arya (#236)

* style: acknowledge sqrtRatio and A!=0 findings

* refactor: SSWU fewer todos. TODO (:p) no isogeny case for A coeff

* style: updated svdw standard reference, no change since previous version

* style: sswu references, no change since latest implementation

* style: update reference for sqrt_ratio q = 3 mod 4, no change

* style: update sqrt_ratio refs for any field, no change

* style: update sgn0 ref and prettify

* style: update hash and encode to curve refs

* style: more reference updates and pretty squaring

* style: decision made to keep variables local to function when possible

* style: more exponent prettification