resource skeds are divided into two parts: front end (all insts) and back end (inst. specific)
each of those are implemented as separate lists, so this iterator wraps around
the traditional list iterator so that an instruction can walk it's schedule but seamlessly
transfer from front end to back end when necessary
replace priority queue with vector of lists(1 list per stage) and place inside a class
so that we have more control of when an instruction uses a particular schedule entry
...
also, this is the 1st step toward making the InOrderCPU fully parameterizable. See the
wiki for details on this process