title: How do I update a section of a Bytes/BytesMut?
body: <p>I have a fixed size buffer in a <code>Bytes</code> struct, and I want to copy some data over the middle of it.</p>
  
  <p>The only thing I can see at the moment would be to take a slice of the beginning, add what I want, and add the slice at the end, but I'm sure this will result in a large copy or two that I want to avoid, I simply need to update the middle of the buffer. Is there a simple way of doing that without using <code>unsafe</code>?</p>