* Test mutant with zombie.
* Allow matches on attr_reader with literal name argument(s)?
* Allow matches on define_method with literal name argument?
* Add some kind of a "do not touch me object" that raises on all messages.
  It can be used to make sure each literal value is touched.
* Replace nil or add "do not touch me object" to literal mutations.
* Add support remaining dynamic literals
* Mutate options on Regexp literals
* Support the numerous Rubinius::AST::SendWithArguments mutations.
* Fix rubinius to allow setting @vcall_style variable in Rubinius::AST::Send nodes.
* Aggregate warnings on missing spec files
* Make sure loader does not change visibility of injected mutants
* Collect emmited mutants per subtree and do not emit the same subtree twice.