Add mapcar builtin, more tests, fixes

src/builtins.c

@@ -182,3 +182,15 @@ struct ucl_object *ucl_builtin_list(struct ucl_state *state, struct ucl_object *
 
     return head;
 }
+
+LISP_FUNC_2(ucl_builtin_mapcar, state, fun, elems) {
+    // TODO: Support arbitrary number of 'elems' lists
+    struct ucl_object *result = ucl_nil_create();
+    FOREACH_LIST(elems, iter, elem) {
+        struct ucl_object *form = ucl_tuple_create(fun, elem);
+        struct ucl_object *value = ucl_evaluate(state, form);
+        UCL_RET_IF_ERROR(value);
+        ucl_list_append(result, value);
+    }
+    return result;
+}