move count upwards to Collection interface

[inkrement]

If I am not wrong, each specific Collection implementation provides a count() method, would be nice to shift it up to the collection interface.

[morrisonlevi]

This detail was intentional at the time it was made. Collection has changed quite a bit over time so I'll re-examine it to see if it still makes sense to separate this out. Basically I did not want to preclude infinite streams from being Collections.