[core.lib] Add basic "pretty" printing to `store_conf'.

The format is as follows:

```
{
  array = {
    1,
    2,
  },
  table = {
    foo = 1,
    bar = 2,
  },
}
```

Its not *really* pretty, but somewhat readable and the algorithm is simple. Idiomatic Lua formatting would be much more complicated as far as I can imagine.