Update memory management and collection structures

Added `AllocationHandler` struct for memory allocation management.
Added `UnsafeArrayPool` class for pooling `UnsafeArray<T>` instances.
Added new `External` option to `Allocator` enum.
Added default constructors for `UnsafeList`, `UnsafeQueue`, and `UnsafeStack` using `Persistent` allocator.
Changed namespace in `AllocationManager` to `Misaki.HighPerformance.Unsafe.Buffer`.
Changed `MemoryLeakException` to use `MemoryLeakExceptionInfo` for better debugging.
Changed constructor behavior in `UnsafeArray` to clarify memory management responsibilities.
Changed `MemoryUtilities` to include null checks in `Free` and `AlignedFree` methods.
Removed unused using directive in `CollectionBenchmark.cs`.
Removed initialization of `AllocationManager` in `Program.cs`.

Misaki.HighPerformance.Unsafe/Buffer/UnsafeArrayPool.cs

@@ -0,0 +1,19 @@
+using Misaki.HighPerformance.Unsafe.Collections;
+
+namespace Misaki.HighPerformance.Unsafe.Buffer;
+
+// TODO: Implement a pool for UnsafeArray<T>.
+public unsafe static class UnsafeArrayPool
+{
+    public static UnsafeArray<T> Rent<T>(int minimalSize)
+        where T : unmanaged
+    {
+        throw new NotImplementedException();
+    }
+
+    public static void Return<T>(UnsafeArray<T> array)
+        where T : unmanaged
+    {
+        throw new NotImplementedException();
+    }
+}