[Truffle] Fix class variables.

* Use LexicalScope since it seems perfect for this.
  RubyCallStack.getCurrentDeclaringModule() is similar,
  but we need to ignore class_eval/exec scoping.
* Module#class_variable_defined? should not recurse.
* Only set in one module in setClassVariable().
* Move Class#class_variables to Module.
* Sort class variables methods in ModuleNodes.