Deprecate zarr.storage

[TomAugspurger]

This deprecates the usage of zarr.storage. For classes where we have a (roughly) equivalent Store in v3, a warning is emitted and the class from the new location is returned:

In [1]: from zarr.storage import ZipStore
<ipython-input-1-f60ad0b873ae>:1: FutureWarning: 'zarr.storage' is deprecated. Import from 'zarr.store' instead.
  from zarr.storage import ZipStore

[jhamman]

@TomAugspurger - this is very cool. However in #2186, I've renamed zarr.store -> storage. I honestly don't think we had good reason to use a different module name.

[TomAugspurger]

Even better :) I was wondering that myself but don't want to rock the boat too much.