[Cs3] iterator arithmetic with deques

[Mikhail Nesterenko]

CS3 students,

I stand corrected. Iterator arithmetic with deques, this includes
adding numbers to, subtracting two iterators, and comparing them for
inequality (< > <= and >=) are all valid and work correctly despite
non-continuous space allocation of deques. All the operators are
appropriately overloaded.

I updated the slides and deque example to reflect that.

thanks,
-- 
Mikhail