CUE Roadmap Alignment

[sdboyer]

To help the CUE folks decide what they ought work on next, we'll use this issue to highlight the things scuemata would most benefit from progress on.

Native CUE subsumption constraint

A critical, top-line goal for scuemata is being able to express all of the invariants required by the system as native CUE constraints. To be able to assert backwards compatibility over a sequence, we need a subsumption operator. (Probably must as well.)

Functions/Macros

Interacting with scuemata from CUE will necessarily involve a number of helper functions, largely analogous to what's in the Go API. I think it'd make for a much better API consumer experience to be able to write these using the new syntactic sugar for functions/macros described in cue-lang/cue#943.

Generated documentation

As a framework for writing schema, Scuemata has a particular need for standardized docs generation. This will, necessarily, be at least somewhat divergent from the more general case for CUE docs generation, as we have a very specific logical structure to document. Still, if there's any scaffolding we could take advantage of, it would be beneficial.

There's also a more specific issue: unification carries comments along with, as it should. But when writing in layers (scuemata def, concrete object, possibly an intermediary like polly), these all get smashed together, even though not all the layers are necessarily appropriate for all generated docs. I suspect this is a fairly general problem, so any system for helping tease apart layers for rendering would be helpful.