Affine(p *PointG1) changes the underlying PointG1 #37

Currently, the Affine function is calling g.affine(p, p):

But then it changes the underlying point p, because of this Set:

This is unexpected from a function that's supposed to return an affine representation, since it actually mutates the point (and can cause nasty race conditions in code assuming the Affine is really just a getter and not a setter.)