Changes on Bags

I implemented an instance of Changing for bags. Of course, I ran into *the* problem n. 1 with associated type families (ambiguity), so I stopped using them.
Say no to associated type families!

Writing the instances needed to test it seems somewhat hard, especially because we already need SmallCheck here: either I use `Integer` for multiplicities and make them unbounded, or otherwise exhaustive testing will find overflow bugs. What do you think?