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.

src/main.c

@@ -23,9 +23,7 @@ int main(void) {
     tokenize(buf, &context);
 
     ParseResult par = parse(&context);
-    AST tree = par.tree;
-    int64_t result = evaluate(tree.head);
-    arena_destroy(par.arena);
+    int64_t result = evaluate(par);
 
 
     printf("El resultado es: %" PRIi64 "\n", result);