[mem cache] separate cache and cache_block data structures - better memory utilization + allow storing metadata in cache structure
[mem cache] cosmetics - rename block create/destroy functions
[mem cache] block size as macro
[mem cache] store cache name for debugging
[mem cache] store mem caches in a list so they can be traversed for debugging
[mem cache] generalize function for getting number of free items so it could be used for statistics
[mem cache] add mem cache statistics
[builtins] move meminfo() to globals.c