[Truffle] Add the possibility to make singleton methods.

* rb_define_singleton_method() in MRI, (meta = true) in JRuby.
* Refactor addMethod().
[Truffle] Use our shiny new singleton methods.

* Now all @CoreMethod should be correctly classified.
* Usage of self is rather rare, so use the same strategy as for isModuleFunction.
Merge branch 'master' into truffle-head