Unions of 3 or more types involving modules are not commutative

[HertzDevil]

#10527 made Union(T, U) and Union(U, T) equivalent only if neither T nor U is itself an (irreducible) union type, as the outer type merge algorithm (Crystal::Program#type_combine) was untouched. Certain union operations still fail to be commutative and associative if unions and modules are involved:

module A; end
class B; include A; end
class C; include A; end

(A | B) | C # => A
(A | C) | B # => A
(B | A) | C # => A
(B | C) | A # => (A | C)
(C | A) | B # => A
(C | B) | A # => (A | C)

A | (B | C) # => A
A | (C | B) # => A
B | (A | C) # => A
B | (C | A) # => A
C | (A | B) # => A
C | (B | A) # => A

Union(A, B, C) # => A
Union(A, C, B) # => A
Union(B, A, C) # => A
Union(B, C, A) # => (A | C)
Union(C, A, B) # => A
Union(C, B, A) # => (A | B)

All of the above unions should produce A.