Refactored Arena, DynamicArena, FreeList, and Stack to implement the new IMemoryAllocator<TSelf, TOpts> interface, standardizing allocation and free methods. Introduced MemoryPool<T, TOpts> for allocator lifetime management and AllocationHandle access. Updated Program.cs to use MemoryPool, replacing AllocationManager. Minor project file updates included.
Added new AllocationHandle property in Stack.Scope.
Changed the ref AllocationHandle constructor parameter to AllocationHandle on of all UnsafeCollection types
Removed Allocator.Stack. Use Stack.Scope.AllocationHandle to allocate on stack instead.
