Make sure we a good definition of Plate values

[seagreen]

Values are currently defined in docs/specification.md.

1. Is the rationale for making values more complicated than type Value = Map String Value solid?

2. Can we find a principled distinction between things like sequences that we define as primitive values and other things like multisets that we don't?

3. Once we find a principle for determining what to make a primitive value, does it suggest any new primitives we should add (or old ones we should take away)?