misc: Replace references to IntPtr/UIntPtr with nint/nuint + code cleanups.

src/ARMeilleure/Memory/ReservedRegion.cs

@@ -8,7 +8,7 @@ namespace ARMeilleure.Memory
 
         public IJitMemoryBlock Block { get; }
 
-        public IntPtr Pointer => Block.Pointer;
+        public nint Pointer => Block.Pointer;
 
         private readonly ulong _maxSize;
         private readonly ulong _sizeGranularity;