`as_sexp_strings()` could use a single `unwind_protect()` around the loop

To convert from `std::vector<std::string>` to `cpp11::strings` you might do `cpp11::strings(as_sexp(x))`. That goes through `as_sexp_strings()` which looks like it could be a little more efficient. Rather than calling `safe[Rf_mkCharCE]` on each iteration, I think we could wrap the whole loop in a single `unwind_protect()`. https://github.com/r-lib/cpp11/blob/05c888b0c6f49e7b252b79b028e4719e6d5b299d/inst/include/cpp11/as.hpp#L287-L306 Something like https://github.com/r-lib/cpp11/blob/05c888b0c6f49e7b252b79b028e4719e6d5b299d/vignettes/FAQ.Rmd#L499-L503 Be careful to avoid all the pitfalls of calling `unwind_protect()` manually that are mentioned in that vignette