implement and use TruffleLanguage.disposeContext

...ine/src/com/oracle/truffle/r/engine/TruffleRLanguage.java

...ngine/src/com/oracle/truffle/r/engine/shell/RCommand.java

...src/com/oracle/truffle/r/engine/shell/RscriptCommand.java

.../src/com/oracle/truffle/r/library/fastr/FastRContext.java

...es.test/src/com/oracle/truffle/r/nodes/test/TestBase.java

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

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

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

.../src/com/oracle/truffle/r/test/generate/FastRSession.java