[clib] Provide `filter_range` function.

This is very useful to compute efficiently a list of prefixes. Will be
used in conjunction with the nametab to provide completion.

Example of use:

```
let cprefix x y = String.(compare x (sub y 0 (min (length x) (length y)))) in
M.filter_range (cprefix "foo") m
```

We could of course maintain a trie, but this is less invasive an
should work at our scale.

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