[ox/std] Remove append operators from IString

This is because the append operators cannot report the failure that is possible with IString

deps/ox/src/ox/std/test/tests.cpp

@@ -6,6 +6,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/.
  */
 
+#include "ox/std/def.hpp"
 #undef NDEBUG
 
 #include <map>
@@ -69,10 +70,10 @@ static std::map<ox::StringView, ox::Error(*)()> tests = {
 		"BString",
 		[]() {
 			ox::IString<5> s;
-			s += "A";
-			s += "B";
-			s += 9;
-			s += "C";
+			oxReturnError(s.append("A"));
+			oxReturnError(s.append("B"));
+			oxReturnError(s.append("9"));
+			oxReturnError(s.append("C"));
 			oxAssert(s == "AB9C", "BString append broken");
 			s = "asdf";
 			oxAssert(s == "asdf", "String assign broken");