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.
Actually, is the check against cap necessary? Wouldn't it amount to:

```
  // [A o o o . . . . . o o o o o o o ]
  // [A o o o . . . . o o o o o o o I ]
```