Make EAs for all Block subclasses

[jbrockmendel]

A Block is basically an Array with mgr_locs attached. An Index is basically an Array with some fancy indexing and set operations. These should share a bunch of their code.

Block.make_block behaves essentially the same way as Index._shallow_copy. blocks.make_block behaves like Index.__new__. Some values vs _values vs _ndarray_values vs _data juggling may be needed to combine these implementations.

For standard dtypes (float, int, uint, ...) these EAs can be thin wrappers around the numpy implementations.

Aside from de-duplicating a bunch of code, this would allow us to standardize on conventions like ".values always returns an EA".

The only change I envision to EA is that this would entail needing to support 2D arrays (which I'd prefer anyway...).