Explain Swift's memory layout and ABI stability.

Swift's memory layout and ABI stability ensure consistent binary interfaces across compiler versions.\n\n• **Memory Layout:**\n• Value types stored inline\n• Reference types use heap allocation\n• Existential containers for protocols\n• Witness tables for dynamic dispatch\n\n• **ABI Stability:**\n• Binary compatibility across Swift versions\n• Stable since Swift 5.0\n• Enables OS-level Swift runtime\n• Backward compatibility guarantees\n\n• **Layout Optimization:**\n\n// Struct layout is optimized\nstruct OptimizedStruct { \n let flag: Bool // 1 byte\n let value: Int64 // 8 bytes, aligned\n let small: Int8 // 1 byte, packed\n}\n\n// Class layout includes metadata\nclass MyClass { \n let isa: UnsafeRawPointer // Class metadata\n let refCount: Int // Reference count\n let property: Int // Actual properties\n}\n\n\n• **Existential Containers:**\n• Store protocol conforming values\n• Inline storage for small values\n• Heap allocation for large values\n• Witness table for method dispatch