Implement multi-tensor parameter save/load facilities

[mdekstrand]

To fully support model saving (#687), we need functions to save parameters, probably in the form of a dictionary mapping names to tensors.

We can't just use safetensors or zarr out of the box, because LensKit supports multiple different tensor backends, and each component needs its tensors in the correct format. We can use one (or both) of these libraries, but with additional metadata on the form a tensor should take when deserialized.

Currently leaning to towards zarr, either directly or via Google TensorStore.