Implement insert for RingBuf

This is a first pass at insert on RingBuf. I tried to keep it as simple as possible. I'm not sure of the performance implications of doing one copy vs. copying multiple times but moving a smaller amount of memory. I chose to stick with one copy, even if the amount of memory I have to move is larger.

I believe this is part of #18424 

@Gankro mentioned this was missing.
You can actually do:

```
let (src, dest, count) = if ... {
   // do some work...
   (x, y, z)
} else {
  // do some work ...
  (a, b, c)
};
```