Consider abandoning DPred

[goldfirere]

Once upon a type, predicates were much simpler than ordinary types. In those halcyon days, it made sense to separate DPred from DType so that code that consumes predicates needn't match on impossible cases.

Those days are long gone now. It's true that we'll never have, say, a literal at the head of a predicate, but that's really a kind error, not a syntax error. And I worry that keeping DPred separate isn't really worth it anymore.

What think you?

[RyanGlScott]

I would support this. There's quite a bit of faffing about in the code to convert from DPreds to DTypes in various places, which feels silly given that DType only differs from DPred in two places.