`DeletePredicate` is a simpler version of `Predicate` that is based on
IOx `DeleteExpr` instead of the full-blown DataFusion `Expr`. This will
allow us to do a couple of things (in follow-up changes):
- Order and de-duplicate delete predicates
- Normalize predicates
- Infallible serialization
- Smaller memory footprint
Note that this change only affects delete expressions. Query expressions
that are supported via the API are not changed. The query subsystem also
still uses the full-featured expressions/predicates (delete
expressions/predicates are converted to the more powerful DataFusion
version on-the-fly).