Add O(1) append to astn_list

`list_append()` becomes a big problem because we need to append very often and the runtime blows up for large input programs. The current append is O(n). For:

```c
int i = 0;
i++; // #1
i++; // #2
i++; // #3 
...
i++; // #40000
```

dcc spends 99.5% of time in `list_append()`, exhibiting O(n^2) runtime for number of `i++`'s.