Data-driven validations and/or code shim

[benkamphaus]

At present, per-entity processing validations in prepare and post-transactional validations are not wired in, as these required bespoke code. This functionality is still important, though. One workaround is just to write validations using normal specs or schema libs, trawling over entity types, and I might write an example of this somewhere, probably in tests, as an example. But many specs in CANDEL were about required attributes, allowed values as sets, min and max values, etc. and it would be easy to encode these in plain edn, either as part of or in addition to the simplified Unify schema edn representation. (Most likely in addition, maybe optionally produced from a shared representation by compile).