Allow formatting C strings as pointers (#223)

test/util-test.cc

@@ -593,6 +593,7 @@ struct TestVisitor : fmt::internal::ArgVisitor<TestVisitor, Result> {
   Result visit_double(double value) { return value; }
   Result visit_long_double(long double value) { return value; }
   Result visit_char(int value) { return static_cast<char>(value); }
+  Result visit_cstring(const char *s) { return s; }
   Result visit_string(Arg::StringValue<char> s) { return s.value; }
   Result visit_wstring(Arg::StringValue<wchar_t> s) { return s.value; }
   Result visit_pointer(const void *p) { return p; }