Fix type stability in multiset_permutations

Author: FedericoStra

src/permutations.jl

@@ -287,7 +287,7 @@ julia> collect(multiset_permutations([1,1,2], 3))
 """
 function multiset_permutations(a, t::Integer)
     m = unique(collect(a))
-    f = [sum([c == x for c in a]) for x in m]
+    f = [sum(c == x for c in a)::Int for x in m]
     multiset_permutations(m, f, t)
 end