Properly calculate the number of mutations

We need to give everything inside a gene (Connections, Constant, Function)
a chance to mutate, not just the genes themselves. While the actual mutations
themselves were correct, the number of mutations was too low.
Use the Symbolic Regression testcase as example in the README instead

It's too slow and often takes longer than the ten minutes allotted to tests, so
it makes the tests fail. However, it's a good example of how to use the library
so I'm putting it in the README.