[Truffle] Implement {Module,Class}#initialize and {class,module}_exec.

* Unnamed modules should not have a lexical parent.
[Truffle] Be safer in MRIBacktraceFormatter with NullSourceSection.
[Truffle] Ensure a RubyMethod has the right declaringModule.
[Truffle] Extract a few helper methods in method definition nodes.
[Truffle] Anonynous classes should have unset parent so the singleton class has the right parent.
[Truffle] Add ModuleOperations.lookupSuperMethod.

* And try to partially fix AbstractGeneralSuperCallNode.