[GR-4259] Refactor FFI impls into a single project.

.../com/oracle/truffle/r/engine/interop/ActiveBindingMR.java

...ruffle/r/engine/interop/RAbstractVectorAccessFactory.java

...c/com/oracle/truffle/r/engine/interop/REnvironmentMR.java

...e/truffle/r/engine/interop/RForeignAccessFactoryImpl.java

...gine/src/com/oracle/truffle/r/engine/interop/RListMR.java

...gine/src/com/oracle/truffle/r/engine/interop/RNullMR.java

.../src/com/oracle/truffle/r/engine/interop/RPairListMR.java

...c/com/oracle/truffle/r/ffi/impl/common/Generic_Tools.java

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

...pl/src/com/oracle/truffle/r/ffi/impl/common/LibPaths.java

...src/com/oracle/truffle/r/ffi/impl/common/ParseResult.java

...l/src/com/oracle/truffle/r/ffi/impl/common/RFFIUtils.java

...cle/truffle/r/ffi/impl/common/TracingUpCallsRFFIImpl.java

...rc/com/oracle/truffle/r/ffi/impl/common/package-info.java

...m/oracle/truffle/r/ffi/impl/interop/CharSXPWrapperMR.java

...com/oracle/truffle/r/ffi/impl/interop/DLLDotSymbolMR.java

.../src/com/oracle/truffle/r/ffi/impl/interop/DLLInfoMR.java

...fle/r/ffi/impl/interop/FFI_RForeignAccessFactoryImpl.java

...om/oracle/truffle/r/ffi/impl/interop/NativeCharArray.java

.../oracle/truffle/r/ffi/impl/interop/NativeCharArrayMR.java