ShapedArray type and/or protocol

Add a ShapedArray type and/or protocol suitable for use as the common currency type for vectors / linear algebra / image processing / tensors, like ndarray is for numpy.

This should hew pretty closely to what S4TF already does with their ShapedArray type; it doesn't need to be identical, but we need to have a good reason for any changes.

Start by comparing the proposed scope with S4TF's ShapedArray type, as requested in the issue. Define whether the result is a type, a protocol, or both, and consider support for vectors, linear algebra, image processing, and tensors; done requires an agreed common currency with justified differences from S4TF.

Written by the indexing model from the issue text.