support for simple multi-tenancy (multiple RContexts)

...gine/src/com/oracle/truffle/r/engine/RContextFactory.java

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

...c/com/oracle/truffle/r/nodes/builtin/RBuiltinPackage.java

.../com/oracle/truffle/r/nodes/builtin/RBuiltinPackages.java

...ltin/src/com/oracle/truffle/r/nodes/builtin/base/Cat.java

...cle/truffle/r/nodes/builtin/base/ConnectionFunctions.java

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

...om/oracle/truffle/r/nodes/builtin/base/EvalFunctions.java

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

...truffle/r/nodes/builtin/base/HiddenInternalFunctions.java

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

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

...in/src/com/oracle/truffle/r/nodes/builtin/base/Parse.java

...racle/truffle/r/nodes/builtin/base/PrettyPrinterNode.java

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

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

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

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

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

...rc/com/oracle/truffle/r/nodes/access/AccessFieldNode.java