various enhancements and bugfixes for the native ffi

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

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

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

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

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

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

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

.../com/oracle/truffle/r/runtime/ffi/jnr/CallRFFIHelper.java

...com/oracle/truffle/r/runtime/ffi/jnr/CallRFFIWithJNI.java

...ntime/src/com/oracle/truffle/r/runtime/gnur/SEXPTYPE.java