rework: AST now uses an arena for allocation

For now it works but i dont really like that i use ParseResult, i mean
is necessary but i think i will try to make it cleaner so that i can
just directly use like parse and pass tath into evaluate, that would
require to move the main evaluate funciton into evaluate_tree or
something  and evaluate takes the arena, uses evaluate_tree and frees
the arena, will try that the next commit but for now this version works
perfectly.

test/test_evaluator.c

@@ -16,7 +16,8 @@ static void test_basic_evaluation(void** state) {
     ASTNodeArray context;
 
     tokenize(expr, &context);
-    AST tree = parse(&context);
+    ParseResult result = parse(&context);
+    AST tree = result.tree;
     uint64_t value = evaluate(tree.head);
 
     assert_int_equal(value, 82);