Instead of iterating *all* swept cells when pruning weak containers,
only iterate the cells actually *in* the container.
Also, instead of compiling a list of all swept cells, we can simply
check the Cell::state() flag to know if something should be pruned.
Make this API take a Span<Cell*> instead of a Vector<Cell*>&.
This is behavior neutral, but stops the API looking like it wants to
do mutable things to the Vector.
