Clean up the DescendantIter type.
Fix naming and lint boo-boos.
Add is_store_overflow_delayed().
Use descendant_iter in two more places.
Use block instead of using self.as_block() several times.

blk -> block everywhere.
Move common functions into the Flow trait.
Make solve_vertical_constraints_abs_position a constructor.
Fix big boo-boo regarding `gives_abs_offsets`. Add reftest.
Extract kid_block in collect_static_y_offsets_from_kids.
Remove unnecessary positioning field from BlockFlow.

Move many obvious methods entirely into Flow.