[GR-2738] Fix various imports and warnings.

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

...c/com/oracle/truffle/r/ffi/impl/llvm/TruffleLLVM_DLL.java

...racle/truffle/r/ffi/impl/upcalls/VariableUpCallsRFFI.java

...om/oracle/truffle/r/nodes/builtin/base/GrepFunctions.java

...tin/src/com/oracle/truffle/r/nodes/builtin/base/Prod.java

.../truffle/r/nodes/builtin/helpers/BrowserInteractNode.java

.../com/oracle/truffle/r/nodes/function/ArgumentMatcher.java

...fle.r.runtime/src/com/oracle/truffle/r/runtime/Utils.java

...src/com/oracle/truffle/r/runtime/conn/StdConnections.java

com.oracle.truffle.r.test.packages/r/install.packages.R

.../oracle/truffle/r/test/library/fastr/TestJavaInterop.java