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.
What is buf here? The ringbuf itself, or its underlying raw buffer? In the raw buffer (what you are indexing into here) index 0 has no particular reason to contain data. This branch is the contiguous case, no?