implement SHARED_CODE contexts, some method renaming

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

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

com.oracle.truffle.r.native/library/fastr/src/R/fastr.R

...om/oracle/truffle/r/nodes/builtin/fastr/FastRContext.java

...cle/truffle/r/nodes/builtin/fastr/FastRFunctionEntry.java

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

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

...me/src/com/oracle/truffle/r/runtime/env/REnvironment.java

...ffle.r.shell/src/com/oracle/truffle/r/shell/RCommand.java

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