cpp11 0.2.5

• cpp_source() gains a cxx_std argument to control which C++ standard is used.
  This allows you to use code from C++14 and later standards with cpp_source(). (#100)
• The cpp11 knitr engine now allows you to set the cxx_std chunk option to control the C++ standard used.
• cpp_source() now has much more informative error messages when compilation fails (#125, #139)
• cpp_source() now uses a unique name for the DLL, so works when run multiple times on the same source file on Windows (#143)
• writable::list_of<T> now supports modification of vectors as intended (#131).
• Errors when running tools::package_native_routine_registration_skeleton() are no longer swallowed (#134)
• cpp_source() can now accept a source file called cpp11.cpp (#133)
• named_arg now explicitly protect their values, avoiding protection issues when using large inputs. tidyverse/readr#1145
• r_string(std::string) now uses Rf_mkCharLenCE() instead of Rf_mkChar(), which avoids the performance cost of checking the string length.
• Writable vector classes now properly set their lengths as intended when being copied to a read only class (#128).