Index types / integer provenance
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It would be nice to be able to use the type system to avoid conversions
between different kinds of integers.

There are many different types and subtypes of integers:

* Indexes of arrays:
    - Subtyped based on element type
    - Could possibly be further subtyped depending on the semantic meaning of
      the particular array.
* Integral units:
    - Subtyped based on "unit" (e.g. "seconds" vs "pixels")
    - Further subtyped based, e.g. "duration" vs "timestamp".