for dbscheme elements with direct or indirect location links in dbscheme.
- Unify discardable entities under one Discardable superclass.
- Two discard predicates depending on TRAP ID type.
- Future-proof the XML and Yaml discard predicates for when their
extractors become incremental.
- new syntactic category `Pattern` (in `Patterns.qll`)
- subpatterns available on statments
- new statements `MatchStmt` and `Case`
(`Match` would conflict with the shared ReDoS library)
- new expression `Guard`
- support for pattern lists
