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.
I would like to see a more comprehensive test here. I'm thinking something along the lines of: for every possible head and tail position in a 64-element ringbuf and every valid insertion position in that range; create the ringbuf, try the insertion, then validate that the contents meet expectations.