Add mutator for self nodes

[dkubb]

This branch adds a mutator for self nodes.

[mbj]

@dkubb I'm not really convinced these are valid mutations. self is never ever nil. A mutation IMHO should force to test for a state that is "reachable". That nil for self is not.

[dkubb]

@mbj what about a bare self? If it were mutated to nil and did not cause a spec failure, then that would probably be dead code.

[mbj]

@dkubb Okay. But dead code should normally be "spotted" by statement deletion. But I realized I'm inconsistent with myself. We also mutate literals to nil. So we can do the same to with self.