Add p_context_delete() and p_tree_delete() for D targets

spec/test_tree_token_positions.d

@@ -10,8 +10,7 @@ int main()
 unittest
 {
     string input = "abbccc";
-    p_context_t * context;
-    context = p_context_new(input);
+    p_context_t * context = p_context_new(input);
     assert(p_parse(context) == P_SUCCESS);
     Start * start = p_result(context);
 
@@ -47,6 +46,8 @@ unittest
     assert_eq(1, start.end_position.row);
     assert_eq(6, start.end_position.col);
 
+    p_tree_delete(start);
+
     input = "\n\n  bb\nc\ncc\n\n     a";
     context = p_context_new(input);
     assert(p_parse(context) == P_SUCCESS);
@@ -83,4 +84,6 @@ unittest
     assert_eq(3, start.position.col);
     assert_eq(7, start.end_position.row);
     assert_eq(6, start.end_position.col);
+
+    p_tree_delete(start);
 }