feature: add support for CtExecutableReference#getOverridingExecutables returning a list of references

Since Java 8 interfaces may have default implementations for methods. Hence, `CtExecutableReference#getOverridingExecutable` should return a list of references in case a method overrides multiple default methods of implemented interfaces. Furthermore, this method (in my opinion) should consider non-default methods declared in interfaces, too---not only methods of super classes and default methods.