Delegate ParLoop caching to Cached base class
Remove helper functions _empty_parloop_cache, _parloop_cache_size
Only cache generated ParLoop stub in JITModule class
Returning None for _cache_key means don't store in cache
Classes derived from Cached need to provide their own _cache
Delegate _GenericPlan caching to Cached base class
Remove helper functions _empty_plan_cache and _plan_cache_size
Cache kernels
Add kernel caching unit tests