Added support for S4 object sharing.

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

...nodes/src/com/oracle/truffle/r/nodes/RTruffleVisitor.java

...oracle/truffle/r/nodes/function/WrapArgumentBaseNode.java

...s/src/com/oracle/truffle/r/nodes/unary/DuplicateNode.java

...time/src/com/oracle/truffle/r/runtime/data/RFunction.java

...time/src/com/oracle/truffle/r/runtime/data/RS4Object.java

...m/oracle/truffle/r/test/builtins/TestBuiltin_unclass.java

...cle/truffle/r/test/library/base/TestSimpleAttributes.java