Replace Array type alias with abstraction

The Array type is just an alias for AMT. This lack of abstraction can lead to issues like #1346. Also, exposing the amt package errors directly causes a bunch of bloat in users that need to wrap and attach exit codes.

Create an `Array2` type following the same pattern as `Map2` and replace all uses of Array. Simplify all the `map_err` wrapping points that result from amt errors.

We can rename `Array2` to `Array` once that name becomes available again.