Cleaning up TIterable hierarchy

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I’m not happy with the organization of TStreamable and the type polymorphism between TArray and TStream. I propose the following:

TStreamable is gone, TArray and TStream now live under TIterable parallel to TSet, etc.

IR nodes take either a stream or array, but not both.

The only operations interact with arrays are: ToArray, ArrayLength, ArrayLen, ArrayRef and ArraySort (maybe others, I haven’t done a careful audit.) Redundant MakeArray and ArrayRange are gone in favor of the corresponding stream functions.

The remaining Array* functions become Stream* functions. The front end is then responsible for inserting the necessary ToArray and ToStream to make the types work out.

Objections?

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I think there’s consensus here as well that this is the right move.

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This PR completes most of the proposed work: https://github.com/hail-is/hail/pull/8171.