fix updated defs not being applied properly to VM in frontend and backend

this fixes the case where

	(def botsbuildbots (fn () (botsbuildbots))) (botsbuildbots)

would not run properly (return with a "set def index out of bounds" error)

also make exceptions store String instead of &'static str for better error reporting

closes #33

crates/haku/tests/language.rs

@@ -167,6 +167,19 @@ fn def_mutually_recursive() {
     expect_number(code, 14.0, 0.0001);
 }
 
+#[test]
+fn def_botsbuildbots() {
+    let result = eval("(def botsbuildbots (fn () (botsbuildbots))) (botsbuildbots)");
+    if let Err(error) = result {
+        assert_eq!(
+            error.to_string(),
+            "Exception {\n    message: \"too much recursion\",\n}"
+        );
+    } else {
+        panic!("error expected");
+    }
+}
+
 #[test]
 fn let_single() {
     let code = r#"