Addes support for S4 objects and external pointers to "inherits" function implementation.

...oracle/truffle/r/library/methods/MethodsListDispatch.java

...n/src/com/oracle/truffle/r/nodes/builtin/base/DoCall.java

...src/com/oracle/truffle/r/nodes/builtin/base/GetClass.java

...rc/com/oracle/truffle/r/nodes/builtin/base/Identical.java

...src/com/oracle/truffle/r/nodes/builtin/base/Inherits.java

...cle/truffle/r/nodes/builtin/base/S3DispatchFunctions.java

...c/com/oracle/truffle/r/nodes/builtin/base/UpdateSlot.java

...src/com/oracle/truffle/r/nodes/access/AccessSlotNode.java

...des/src/com/oracle/truffle/r/nodes/control/BlockNode.java

...m/oracle/truffle/r/nodes/function/ClassHierarchyNode.java

...le/truffle/r/nodes/function/ClassHierarchyScalarNode.java

...om/oracle/truffle/r/nodes/function/GroupDispatchNode.java

...es/src/com/oracle/truffle/r/nodes/function/RCallNode.java

...racle/truffle/r/nodes/function/UseMethodInternalNode.java

...es/src/com/oracle/truffle/r/nodes/unary/InheritsNode.java

...me/src/com/oracle/truffle/r/runtime/context/RContext.java

.../oracle/truffle/r/test/builtins/TestBuiltin_inherits.java