Idea: LRUCache - similar to StrictMap

[TysonAndre]

• new LRUCache(?int $maxSize = null)
• Make unshift() operations faster, track the offset of the start of the buckets for LRUCache, to speed up iteration and removal
• Add $cache->getOrCreate(mixed $key, callable $create)
• Benchmark difference from polyfills, e.g. for strings, objects, etc.

[TysonAndre]

Adding getOrCreate($key, callable $callback) to Map seems easier.

Tracking start HashPosition would avoid this worst-case performance in maps in general (update on iteration)