API: Remove IntervalArray from top-level
cc @jschendel @jreback @jorisvandenbossche 

Originally, we put IntervalArray in the top-level since it has the special alternative constructors. But it's a bit inconsistent with all our new arrays, so this is a proposal to remove it from the top-level and just have it in `pandas.arrays`. I don't think the extra typing is too bad, and it's more consistent with our other (new) arrays.