Complete the data serialization facility

[Ravenwater]

When working with tensors of custom number types, serializing these tensors to communicate or store them is a key requirement.

Serialization of custom number encodings can be efficient only if we leverage the structure exposed by aggregation. The conceptual problem is that data structures are not Universal's purview, so building an external-facing facility for a stream or file format of Universal types is unlikely to be useful. That has to be solved by the aggregate data structure layers, examples being Arrow or HDF5.

What we can do is build a facility that allows Universal tests to serialize state for testing. This will help the quality of the library, and it could be a reference point for more capable serialization formats like Arrow and HDF5.

Completing this task will resolve #509