DLL mods for multiple contexts; more fastr_context programmaibility

...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/NAMESPACE

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

...e.r.runtime/src/com/oracle/truffle/r/runtime/ffi/DLL.java