[Truffle] newLexicalVersion() only if actually changing the visibility.
[Truffle] Implement private constants and pass all their specs.

* The ReadConstantNode is now passed its LexicalScope.
  We still need to have it in SharedMethodInfo only for Module.nesting (if aliased).
* Add an RubyModule.includedModules() iterator,
  as ancestors() but stopping at the first class seen.
[Truffle] Pass the context explicitly to constant lookup functions.
[Truffle] Fix special lookup for the module keyword.