Add scalar multiplication for multisets.

The operation n *: X multiplies the multiplicity of each element x ∈ X with n

This MR:

• Introduces a type class ScalarMul N A := N → A → A with associated notations for scalar multiplication. The notation *: is taken from ssreflect, see https://github.com/math-comp/math-comp/blob/master/mathcomp/ssreflect/ssrnotations.v
• Provides an instance ScalarMul nat (gmultiset A).
• Provides a bunch of lemmas for *: on multisets
• Extends multiset_solver and adds a couple of tests.