implement VM::call_super -> rb_call_super

[askreet]

This patch adds a way to call super from a method. My use case is ensuring superclass initialize is called from a module that also defines an initialize of it's own (whether or not that's a good idea notwithstanding).

Not sure where to start in writing doc/test for this, open to pointers on next steps before acceptance.

[danielpclark]

This is now in the master branch and Rutie crate 0.8.2