fix/refactor: Modified evaluate and changed it to evaluate_tree

So i did what the last commit said, also fixed parse_expr because it was
still using malloc for allocating new nodes so i made it use arena_alloc
like it should, did the very first tests so it's all good, i think is
readdy to merge.

test/test_evaluator.c

@@ -17,8 +17,7 @@ static void test_basic_evaluation(void** state) {
 
     tokenize(expr, &context);
     ParseResult result = parse(&context);
-    AST tree = result.tree;
-    uint64_t value = evaluate(tree.head);
+    int64_t value = evaluate(result);
 
     assert_int_equal(value, 82);
 }