checking for named_arg

TODO: The equivalent ctor from `initializer_list<r_string>` has a specialization for `<r_string>` to translate `elt` to UTF-8 before assigning. Should we have that here (https://github.com/r-lib/cpp11/blob/main/inst/include/cpp11/r_vector.hpp#L879) too? `named_arg` doesn't do any checking here.
