New CRAN warning: identifier preceded by whitespace in a literal operator declaration

Hi All, CRAN has started flagging a new significant warning in the cpp11 headers: ``` Found the following significant warnings: ../inst/include/cpp11/R.hpp:54:31: warning: identifier '_xl' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator] ../inst/include/cpp11/named_arg.hpp:44:29: warning: identifier '_nm' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator] ``` Here's the relevant [checking and install log](https://www.r-project.org/nosvn/R.check/r-devel-linux-x86_64-fedora-clang/QuickJSR-00check.html) The fix is simple enough (changing the declaration from `operator"" _xl` to `operator""_xl`), but just flagging here for awareness Thanks!