Add dump_to and dump_init functions

These functions would take values from an IndexMap.t and put them in a concrete data structure.

```
let dump index indices set c =
Array.iteri (fun offset i -> set c offset (IndexMap.get index i)) indices
```

and something similar which takes an init or fold function and returns the dumped data structure.

The issue names IndexMap.t, IndexMap.get, and Array.iteri but no file or test. Start by locating those entry points and determine the intended concrete data structures and init or fold semantics; done means the dump and dump_init APIs have agreed behavior and documented results.

Written by the indexing model from the issue text.