sets: do some standardising

Signed-off-by: Peter Williams <pwil3058@bigpond.net.au>
sets: add set_intersection() operation

Signed-off-by: Peter Williams <pwil3058@bigpond.net.au>
sets: add symmetric_set_difference() operation

Signed-off-by: Peter Williams <pwil3058@bigpond.net.au>
sets: add set_difference() operation

Signed-off-by: Peter Williams <pwil3058@bigpond.net.au>
sets: add disjoint() function

Signed-off-by: Peter Williams <pwil3058@bigpond.net.au>
sets: add contains() operation for sorted lists

Signed-off-by: Peter Williams <pwil3058@bigpond.net.au>