Specify __all__ in __init__.py

[BrianPugh]

Allows for better autocompletion in IDEs. Also substantially reduces namespace pollution for people that (poorly) use from littlefs import *.

[BrianPugh]

@jrast what do you think of this? I set __all__ so that autocomplete can pick up the API better, and I also flattened out the namespace for all the major classes to meet what a user would expect (mostly for type annotations). I didn't do it for the c-style API, since that's intended to be used as from littlefs import lfs.

[jrast]

I'm fine with this change! I think this would make the public interface more clear. If there are some examples in the docs importing form some subpackage / module, the examples should also be updated.