add an experimental module

[d-v-b]

We should have a module for new, additive features that are potentially useful but also susceptible to major design changes or even removal.

I think the caching store proposed in #3366 falls into this category -- I could see many different outcomes with that feature -- maybe caching becomes basic functionality for all zarr stores, or maybe the cache store gets spun off into its own library.

Experience with this feature would be super valuable, but we need to gain that experience safely, in a way that's visible to users, (ideally without more annoying warnings!). So I offer two ideas:

1. A top-level zarr.experimental module that contains all the experimental stuff. The cache store would be exported via zarr.experimental.CacheStore.
2. A storage-level zarr.storage.experimental module that contains experimental storage APIs. The cache store would be exported via zarr.storage.experimental.CacheStore

I don't feel strongly about either of these ideas, but I would like to pick one, implement it, and get #3366 shipped as soon as we can.

[shoyer]

I think either of these are fine and unambiguous.

I lean towards starting with a top-level zarr.experimental and adding internal structure as needed.