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

[DavisVaughan]

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().

cpp11/inst/include/cpp11/as.hpp

Lines 287 to 306 in 05c888b

	SEXP as_sexp_strings(const Container& from, AsCstring&& c_str) {
	R_xlen_t size = from.size();
	SEXP data;
	try {
	data = PROTECT(safe[Rf_allocVector](STRSXP, size));
	auto it = from.begin();
	for (R_xlen_t i = 0; i < size; ++i, ++it) {
	SET_STRING_ELT(data, i, safe[Rf_mkCharCE](c_str(*it), CE_UTF8));
	}
	} catch (const unwind_exception& e) {
	UNPROTECT(1);
	throw e;
	}
	UNPROTECT(1);
	return data;
	}
	} // namespace detail

Something like

cpp11/vignettes/FAQ.Rmd

Lines 499 to 503 in 05c888b

	cpp11::unwind_protect([&] {
	for (R_xlen_t i = 0; i < size; ++i) {
	(void) STRING_ELT(data, i);
	}
	});

Be careful to avoid all the pitfalls of calling unwind_protect() manually that are mentioned in that vignette