Cleanup contract comment string generation `get_source`

In `get_source` in `input.c`:

```cpp
  char buf[1024 + 4]{};
  char *res = buf;
  size_t len = 1024;
```
should use obstacks.

```cpp
      int s = expstart.column - 1;
      int l = expend.column - expstart.column + 1;
 ```
could be `int l = expend.column - s;` and then the if could probably be simplified as well.
