additional TO_NATIVE handlers, support c++ compilation for llvm, use node for ffi unwrap operation, convert scalars to vectors at native boundary, use proper nodes for interop messages

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Refactor ffi impls into com.oracle.truffle.r.ffi.impl project; add ffi AP; TruffleLLVM all RFFI interfaces implemented; no JNI

* Add @TruffleBoundaries to File/Fifo Connections
* Means to disable AWT support in FastR Grid implementation
* Means to turns off loading of native code of builtin packages.
* Managed implementation/mock of RFFIFactory