Handle reverse ranges

This is really ugly and should be refactored.

src/tests.rs

@@ -590,10 +590,15 @@ fn non_string_in_string() -> TestResult {
 }
 
 #[test]
-fn int_in_range() -> TestResult {
+fn int_in_inc_range() -> TestResult {
     run_test(r#"1 in -4..9.42"#, "true")
 }
 
+#[test]
+fn int_in_dec_range() -> TestResult {
+    run_test(r#"1 in 9.42..-4"#, "true")
+}
+
 #[test]
 fn int_in_exclusive_range() -> TestResult {
     run_test(r#"3 in 0..<3"#, "false")