Some binary operators are missing

[saraedum]

there is, for example no mpz_class * Element operator.

We should make sure that all operators are free (see #153) and make Element inherit from multiplicative<mpz_class> and multiplicative<mpq_class> instead of multiplicative<ElementClass>.

We also need to provide an explicit operator*(Element, T) and operator(T, Element).

See for some implementation tricks https://www.boost.org/doc/libs/1_69_0/libs/utility/operators.htm