runtime: Add CopyBlock/InitBlock overloads for ref byte

class Unsafe
{
   void CopyBlock(ref byte destination, ref byte source, uint byteCount);
   void CopyBlock(ref byte destination, ref byte source, UIntPtr byteCount);
   void CopyBlockUnaligned(ref byte destination, ref byte source, uint byteCount);
   void CopyBlockUnaligned(ref byte destination, ref byte source, UIntPtr byteCount);
   void InitBlock(ref byte destination, uint byteCount);
   void InitBlock(ref byte destination, UIntPtr byteCount);
   void InitBlockUnaligned(ref byte destination, uint byteCount);
   void InitBlockUnaligned(ref byte destination, UIntPtr byteCount);
}

Thin wrappers over the cpblk and initblk IL instructions to make them accessible from C#. Needed to implement Span<T>.CopyTo() efficiently.

Since this is intended to be a wrapper around the “cpblk” IL instruction, the behavior is undefined if the ranges overlap. (This suggests Span<T>.CopyTo() needs to handle three cases:

• No overlap - Use Unsafe.CopyTo<T>() - enjoy the perf.
• Overlap one way - Use forward for loop.
• Overlap the other way - Use backward for loop.