REF: Back IntervalArray by array instead of Index
The benefit I have in mind here is that we could back it by a single 2xN array and a) avoid the kludge needed to make `__setitem__` atomic, b) do a view to get native types for e.g uniqueness checks, c) possibly share some methods with NDarrayBackedExtensionArray.

Also just in principle having EAs not depend on Index is preferable dependency-structure-wise.

cc @jschendel 