abstract the representation of external symbols

.../src/com/oracle/truffle/r/library/graphics/RGraphics.java

...oracle/truffle/r/library/methods/MethodsListDispatch.java

com.oracle.truffle.r.native/fficall/src/jni/Rinternals.c

com.oracle.truffle.r.native/fficall/src/jniboot/jniboot.c

...oracle/truffle/r/nodes/builtin/base/DynLoadFunctions.java

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

...com/oracle/truffle/r/nodes/builtin/base/foreign/DotC.java

...ruffle/r/nodes/builtin/base/foreign/ForeignFunctions.java

...om/oracle/truffle/r/runtime/ffi/generic/Generic_Grid.java

...m/oracle/truffle/r/runtime/ffi/generic/Generic_Tools.java

...e.ffi/src/com/oracle/truffle/r/runtime/ffi/jni/JNI_C.java

...fi/src/com/oracle/truffle/r/runtime/ffi/jni/JNI_Call.java

...ffi/src/com/oracle/truffle/r/runtime/ffi/jni/JNI_DLL.java

...src/com/oracle/truffle/r/runtime/ffi/jni/JNI_PkgInit.java

...com/oracle/truffle/r/runtime/ffi/jni/JNI_RFFIFactory.java

...i/src/com/oracle/truffle/r/runtime/ffi/jni/JNI_Stats.java

...src/com/oracle/truffle/r/runtime/ffi/jni/JNI_UserRng.java

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

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

...me/src/com/oracle/truffle/r/runtime/conn/RConnection.java