Open-sourcing F14 for faster, more memory-efficient hash tables

Introduced F14, a 14-way probing hash table implementation within the Folly C++ library, designed for improved performance and memory efficiency over previous specialized hash table implementations. F14 utilizes vector instructions (SSE2/NEON) for parallel intra-chunk searching and a novel reference-counted tombstone strategy (overflow bits) to handle collisions and erasures efficiently. It offers multiple memory layouts (F14FastMap/Set, F14NodeMap/Set, F14Vector) to cater to different use cases, particularly optimizing for scenarios without long-lived references to entries.