make indexing with a UnitRange return StaticArrays?

I think it would be useful for indexing with UnitRanges to produce StaticArrays instead of dynamically sized arrays. The current behavior means that something like a[2:end] can't be written in an efficient and generic way without having to add StaticArrays as an explicit dependency. I suspect that in almost all cases, constant propagation is smart enough to infer the size of the resulting array at compile time, so if used appropriately, this shouldn't actually introduce any type instabilities. This behavior would also be consistent with how Base handles indexing for tuples (although the implementation was not always well optimized until recently). I am not sure this would qualify as a minor change though, so unsure if this can be changed in 1.x.

No files, tests, or entry points are identified. Begin by locating the existing UnitRange indexing implementation and its tests, then determine whether returning StaticArrays is compatible with the 1.x API; done would require an agreed design and coverage for the changed behavior.