add as_span overload for basic_string which doesn't have nonconst .data() like other contiguous containers

tests/span_tests.cpp

@@ -203,6 +203,15 @@ SUITE(span_tests)
 			overloaded_func(av3.as_span(dim<>(1), dim<3>(), dim<5>()), 't');
 		}
 
+		{
+			string str;
+			span<char> strspan = as_span(str);
+			(void)strspan;
+			const string cstr;
+			span<const char> cstrspan = as_span(cstr);
+			(void)cstrspan;
+		}
+
 		{
 			int a[3][4][5];
 			auto av = as_span(a);