Faster reduction of 2^b + c moduli

[unzvfu]

The base field moduli used with elliptic curves are often chosen to be of the form 2^b + c for some small c; this is the case for the Tweedledee/Tweedledum curves, for example. We can exploit this form of modulus to speed up the reduction algorithm.

[unzvfu]

An implementation of the candidate fast reduction for the Tweedle* curves can be found in #98. Unfortunately it is not faster than the original Montgomery reduction (yet). Closing this issue as the attempt was made and we are now moving on.