[GR-2738] Truffle API usage simplifications.

...fle.r.engine/src/com/oracle/truffle/r/engine/REngine.java

...rc/com/oracle/truffle/r/engine/RRuntimeASTAccessImpl.java

...src/com/oracle/truffle/r/engine/TruffleRLanguageImpl.java

.../com/oracle/truffle/r/engine/interop/ActiveBindingMR.java

...ruffle/r/engine/interop/RAbstractVectorAccessFactory.java

...c/com/oracle/truffle/r/engine/interop/REnvironmentMR.java

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

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

...gine/src/com/oracle/truffle/r/engine/interop/RListMR.java

...e/src/com/oracle/truffle/r/engine/interop/RPromiseMR.java

.../src/com/oracle/truffle/r/engine/interop/RS4ObjectMR.java

...gine/src/com/oracle/truffle/r/engine/shell/REmbedded.java

.../com/oracle/truffle/r/launcher/JLineConsoleCompleter.java

.../src/com/oracle/truffle/r/nodes/test/SpecialCallTest.java

...st/src/com/oracle/truffle/r/nodes/test/TestUtilities.java

...e/src/com/oracle/truffle/r/runtime/RRuntimeASTAccess.java

.../src/com/oracle/truffle/r/runtime/context/EvalThread.java

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

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

....runtime/src/com/oracle/truffle/r/runtime/env/RScope.java