introduce Utils.isInterned

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

...tin/src/com/oracle/truffle/r/nodes/builtin/base/Slot.java

...fle.r.nodes/src/com/oracle/truffle/r/nodes/RASTUtils.java

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

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

.../truffle/r/nodes/objects/CollectGenericArgumentsNode.java

....runtime/src/com/oracle/truffle/r/runtime/RArguments.java

...fle.r.runtime/src/com/oracle/truffle/r/runtime/Utils.java

...me/src/com/oracle/truffle/r/runtime/data/RAttributes.java

...e/src/com/oracle/truffle/r/runtime/data/RDataFactory.java