MINOR: filters: Add per-side filter-sequence constraints

MINOR: filters: Add per-side filter-sequence constraints

Add filter-sequence request/response entries using class[/id] references and
':before()' or ':after()' to place a instance before or after a class or
another instance. Store source locations and inherit entries from defaults
in configuration order.

Apply the constraints before flattening the class trees. Ignore absent
source instances and sources without the selected side, but validate
reference entities and require ids for ambiguous source instances. Check for
loops after each move and report the originating directive on failure.

Keep sequence parsing, application, inheritance and cleanup together so the
ordering feature can be reviewed independently of instance parsing and state
selection. The resulting order is not yet used for stream attachment.