Make the memory handlers readonly structs.

src/libraries/System.Private.CoreLib/src/System/IO/RandomAccess.Windows.cs

@@ -453,13 +453,13 @@ private interface IMemoryHandler<T>
             MemoryHandle Pin(in T memory);
         }
 
-        private struct MemoryHandler : IMemoryHandler<Memory<byte>>
+        private readonly struct MemoryHandler : IMemoryHandler<Memory<byte>>
         {
             public int GetLength(in Memory<byte> memory) => memory.Length;
             public MemoryHandle Pin(in Memory<byte> memory) => memory.Pin();
         }
 
-        private struct ReadOnlyMemoryHandler : IMemoryHandler<ReadOnlyMemory<byte>>
+        private readonly struct ReadOnlyMemoryHandler : IMemoryHandler<ReadOnlyMemory<byte>>
         {
             public int GetLength(in ReadOnlyMemory<byte> memory) => memory.Length;
             public MemoryHandle Pin(in ReadOnlyMemory<byte> memory) => memory.Pin();