group: CofactorGroup::mul_by_cofactor -> CofactorGroup::clear_cofactor

The generic API now only guarantees that the torsion component is
cleared deterministically; group elements may be multiplied by multiples
of the cofactor (not necessarily the actual cofactor), as long as the
choice of multiplier is fixed for a given implementation.

src/groth16/tests/dummy_engine.rs

@@ -399,7 +399,7 @@ impl PrimeGroup for Fr {}
 impl CofactorGroup for Fr {
     type Subgroup = Fr;
 
-    fn mul_by_cofactor(&self) -> Self::Subgroup {
+    fn clear_cofactor(&self) -> Self::Subgroup {
         *self
     }