Convert Array.CopySlow to managed

src/coreclr/System.Private.CoreLib/src/System/Runtime/CompilerServices/CastHelpers.cs

@@ -209,7 +209,7 @@ internal static unsafe class CastHelpers
         [DebuggerHidden]
         [StackTraceHidden]
         [DebuggerStepThrough]
-        private static object? ChkCastAny(void* toTypeHnd, object? obj)
+        internal static object? ChkCastAny(void* toTypeHnd, object? obj)
         {
             CastResult result;
 
@@ -386,7 +386,7 @@ internal static unsafe class CastHelpers
         [DebuggerHidden]
         [StackTraceHidden]
         [DebuggerStepThrough]
-        private static ref byte Unbox(void* toTypeHnd, object obj)
+        internal static ref byte Unbox(void* toTypeHnd, object obj)
         {
             // this will throw NullReferenceException if obj is null, attributed to the user code, as expected.
             if (RuntimeHelpers.GetMethodTable(obj) == toTypeHnd)

src/coreclr/System.Private.CoreLib/src/System/Runtime/CompilerServices/RuntimeHelpers.CoreCLR.cs

@@ -622,7 +622,7 @@ internal unsafe struct TypeHandle
         /// <summary>
         /// The address of the current type handle object.
         /// </summary>
-        private readonly void* m_asTAddr;
+        internal readonly void* m_asTAddr;
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public TypeHandle(void* tAddr)