Replacement handler

cache_create fix:
Initialized usagectr, fwdptr, and fullbit. Made them not pointers in
cache.h

cache_access:
Replacement policy populates cache. I temporarily switch to FIFO if the
line isn’t full in order to fill that line. Otherwise it randomly
selects which line to run replacement on.

Hit handler and replacement policy should work unchanged- we’re just
giving it the line to operate on.