Async object store support

[kylebarron]

Change list

• Add async methods for each existing function.
• Remove superfluous vec clone. PyBytes::new(py, &Bytes::to_vec()) had two clones, I believe, where only one was needed.
• Automatically convert to PyBytes from Cow<[u8]> instead of converting manually. At least for the async version this was a lot easier; I struggled with manually constructing PyBytes objects inside the async closure.

Another option for the API would be to have a separate class AsyncObjectStore that has only the async methods. This might keep the methods cleaner (we wouldn't need to have an async suffix on each method). But alternatively, there's no reason why someone couldn't use the same store for both synchronous and async calls.

This is implemented on top of #5, so ignore the first several commits. It's expected for #5 to be merged first, and then this PR can be squashed.

Closes #2

[ion-elgreco]

This looks interesting! Any chance to get this merged and released? @roeap

[simonbutt]

Any progress getting this released?

[dark0dave]

No progress here. The store API has managed to mature a bit so if someone wanted to come and pick this up (@kylebarron or otherwise), now would be a good time.

I have picked it up.

[ion-elgreco]

Maybe it's easier to fork, and then merge all open PRs into that fork and release it

[roeap]

Thanks for taking care of this @kylebarron! And sorry for letting this sit for so long.