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.
There might be a magic ratio here. Something like the double copy is worth it if the single copy would move twice as much, but I dunno. For now I'd just pick the minimal, and we can dig into optimization later (unless you're into that sort of thing and want to nail it now).