Adopt Python array API standard

[brandonwillard]

We should consider adopting the Python array API standard standard.

I also noticed that the standard uses the terms "singleton dimension" for what we're calling "broadcastables" (i.e. TensorType.broadcastable) and "rank" for our "ndim" (i.e. Type.ndim).
At the very least, we should deprecate Theano's custom terminology in favor of something with more potential for general adoption, like these terms.

[ricardoV94]

They seem to use ndim in this part of the doc https://data-apis.org/array-api/latest/API_specification/array_object.html#attributes

Otherwise, following their API wherever we deviate makes sense. Less things for us to mull over.

[brandonwillard]

@dgerlanc mentioned this example article recently; it provides some examples of how the interface is intended to be used and its relevance to GPU support.