Align ExtensionArray with abc.Sequence
ref https://github.com/pandas-dev/pandas/pull/28762#issuecomment-537760065

Right now the ExtensionArray is missing a few methods that would allow it to align with the `abc.Sequence` interface, namely `count`, `index`, `__reversed__` and `__contains__`

I think we might want to add those items to the interface to aid our type checking and make for more intuitive use cases of the ExtensionArray