Think about refactoring `NamedArrays` and its descendants

The existing `NamedArray` and its derived types `LoopyCallResult` and `NamedCallResult` currently have some inconsistencies in their implementations, including:

* The `expr` property only applies to `NamedArray` and its associated container `DictOfNamedArrays`, not the other two types, as `LoopyCall` and `Call` don't contain corresponding arrays to point to.
* `NamedArray` and `LoopyCallResult` are allowed to be tagged, but `NamedCallResult` isn't.

This could stand to be looked at. One idea would be to try to remove `LoopyCallResult` and `NamedCallResult` and shift any special mapper logic associated with them into `map_loopy_call` and `map_call`. Then potentially `NamedArray` could be left as a dumb generic reference type with no tagging, etc. It's not completely clear to me whether this could work, but it seems worth exploring at some point.