* Allow doing away with CacheItem
Now that we have working FFI finalizers on BBs, it's mostly useless overhead.
We only keep it for DocCache, because it's slightly larger, and memory pressure might put us in a do or die situation where waiting for the GC might mean an OOM kill.
* Expose's LRU slot-only mode
And use it for CatalogCache, which doesn't care about storage space
* Make GlyphCache slots only (storage space is insignificant here, it was
always going to be evicted by running out of slots).
* More informative warning when we chop the cache in half
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