`Affine(p *PointG1)` changes the underlying PointG1

[AnomalRoil]

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

bls12-381/g1.go

Lines 336 to 338 in 3a0f255

	func (g *G1) Affine(p *PointG1) *PointG1 {
	return g.affine(p, p)
	}

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

bls12-381/g1.go

Lines 352 to 355 in 3a0f255

	} else {
	r.Set(p)
	}
	return r

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.)