Improve unification for redundant unions and multiple inheritance.

[gvanrossum]

There really are two fixes to join.py here:

• In TypeJoinVisitor.visit_union_type(), call UnionType.make_simplified_union() so that the union is normalized. Without this I can demonstrate unnormalized unions in error messages. case testUnificationRedundantUnion tests this.
• In join_instances_via_supertype(), previously it would only consider the first base, I now make it consider all bases and pick the result with the longest MRO (breaking ties by using the earlier base). The other tests demonstrate this.

[gvanrossum]

(I ran into this while investigating #1094, but the key issue there is different, and the fix here doesn't change the problem with empty lists. So this is really a separate issue. @JukkaL let me know of you want any changes here.)

[JukkaL]

Looks good to me. One issue though, that we discussed yesterday: I believe that join_types(a, b) may be different from join_types(b, a), which may result in some unpredictability, but let's not worry about it until we encounter a real-world issue caused by this.