Major speed boost for SortedSet#drop and SortedSet#take

Thanks to Dov Murik for the suggestion. This also fixes a bug whereby the
SortedSet order would be lost after taking/dropping a subset.
Add tests to guard against regression in SortedSet#take/#drop

a6673ab fixed bugs whereby #take/#drop did not preserve custom ordering.
These tests ensure that the same bugs will not happen again.