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);
 }

test/test_parser.c

@@ -19,58 +19,57 @@ static void test_parsing_basic_expression(void **state) {
     assert_int_equal(tokens.len, 7);
 
     ParseResult result = parse(&tokens);
-    AST tree = result.tree;
     // Assert head is +
-    assert_int_equal(tree.head->type, NODE_BINARY_OP);
-    assert_int_equal(tree.head->data.binary.op, OP_ADD);
+    assert_int_equal(result.tree->type, NODE_BINARY_OP);
+    assert_int_equal(result.tree->data.binary.op, OP_ADD);
 
-    assert_int_equal(tree.head->data.binary.left->type, NODE_INTEGER);
-    assert_int_equal(tree.head->data.binary.left->data.integer, 2);
+    assert_int_equal(result.tree->data.binary.left->type, NODE_INTEGER);
+    assert_int_equal(result.tree->data.binary.left->data.integer, 2);
 
 
     assert_int_equal(
-            tree.head->data.binary.right->type,
+            result.tree->data.binary.right->type,
             NODE_BINARY_OP
             );
     assert_int_equal(
-            tree.head->data.binary.right->data.binary.op, 
+            result.tree->data.binary.right->data.binary.op, 
             OP_MUL
             );
 
     assert_int_equal(
-            tree.head->data.binary.right->data.binary.right->type,
+            result.tree->data.binary.right->data.binary.right->type,
             NODE_INTEGER);
     assert_int_equal(
-            tree.head->data.binary.right->data.binary.right->data.integer,
+            result.tree->data.binary.right->data.binary.right->data.integer,
             789);
 
     assert_int_equal(
-            tree.head->data.binary.right->data.binary.left->type, 
+            result.tree->data.binary.right->data.binary.left->type, 
             NODE_BINARY_OP
             );
     assert_int_equal(
-            tree.head->data.binary.right->data.binary.left->data.binary.op, 
+            result.tree->data.binary.right->data.binary.left->data.binary.op, 
             OP_DIV
             );
 
     assert_int_equal(
-            tree.head->data.binary.right->data.binary.left->data.binary.right->type, 
+            result.tree->data.binary.right->data.binary.left->data.binary.right->type, 
             NODE_INTEGER
             );
     assert_int_equal(
-            tree.head->data.binary.right->data.binary.left->data.binary.right->data.integer, 
+            result.tree->data.binary.right->data.binary.left->data.binary.right->data.integer, 
             66
             );
 
     assert_int_equal(
-            tree.head->data.binary.right->data.binary.left->data.binary.left->type, 
+            result.tree->data.binary.right->data.binary.left->data.binary.left->type, 
             NODE_INTEGER
             );
     assert_int_equal(
-            tree.head->data.binary.right->data.binary.left->data.binary.left->data.integer, 
+            result.tree->data.binary.right->data.binary.left->data.binary.left->data.integer, 
             3
             );
-    arena_destroy(result.arena);
+    arena_destroy(&result.arena);
 }
 
 int main(void) {