check and fixed code examples

[nicolaihess]

• the primitive and method for instVarAt: changed, this needs some update on the text as well and as the original example included some useful smalltalk fallback code, I added another example that has fallback code
• updated some code output (changed number of results, BoxFloat instead of Float.
• some SystemNavigation calls are different
• MethodReference is gone (I use RGMethodDefinition instead)