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.
Elements don't move in the head half. The elements in the tail half are moved i - 1, and then the element at 0 is moved to cap - 1. I'm not sure how to show it better?