NFI based Connections.c implementation

...oracle/truffle/r/ffi/impl/common/JavaUpCallsRFFIImpl.java

...com/oracle/truffle/r/ffi/impl/upcalls/StdUpCallsRFFI.java

...oracle.truffle.r.native/fficall/src/common/rffi_upcalls.h

...truffle.r.native/fficall/src/truffle_common/Connections.c

.../com/oracle/truffle/r/runtime/conn/NativeConnections.java

...fle.r.test.native/packages/testrffi/testrffi/R/testrffi.R

...uffle.r.test.native/packages/testrffi/testrffi/src/init.c

...e.r.test.native/packages/testrffi/testrffi/src/testrffi.c

...e.r.test.native/packages/testrffi/testrffi/src/testrffi.h

...est.native/packages/testrffi/testrffi/tests/connections.R