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.
Instead of this nested if-else structure, is it possible to use a single match on a triple? That might help decrease the indentation here, and make it look really nice when you split up the docs case-by-case.