ruby: Change PerfectCacheMemory::lookup to return pointer

ruby: Change PerfectCacheMemory::lookup to return pointer

CacheMemory and DirectoryMemory lookup functions return pointers to entries
stored in the memory. Bring PerfectCacheMemory in line with this convention,
and clean up SLICC code generation that was in place solely to handle
references like that which was returned by PerfectCacheMemory::lookup.