[GR-2202] Reference classes and S4 testing.

.../src/com/oracle/truffle/r/engine/interop/RFunctionMR.java

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

...com/oracle/truffle/r/nodes/builtin/base/EnvFunctions.java

...n/src/com/oracle/truffle/r/nodes/builtin/base/R/do.call.R

...com/oracle/truffle/r/nodes/builtin/base/SeqFunctions.java

.../oracle/truffle/r/nodes/builtin/base/StandardGeneric.java

...com/oracle/truffle/r/nodes/builtin/base/UpdateSubstr.java

.../oracle/truffle/r/nodes/access/BaseWriteVariableNode.java

...e/truffle/r/nodes/access/WriteLocalFrameVariableNode.java

...e/truffle/r/nodes/access/WriteSuperFrameVariableNode.java

...uffle/r/nodes/access/variables/LocalReadVariableNode.java

...le/truffle/r/nodes/access/variables/ReadVariableNode.java

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

...es/src/com/oracle/truffle/r/nodes/objects/LoadMethod.java

...rc/com/oracle/truffle/r/runtime/PrimitiveMethodsInfo.java

...le.r.runtime/src/com/oracle/truffle/r/runtime/RError.java

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

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

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

...untime/src/com/oracle/truffle/r/runtime/data/RVector.java