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.
Do you mean documentation on distance_to_tail, distance_to_head and the two booleans I use to choose whether to move the tail or head?

I will run some tests on the push methods vs. doing the buffer_write directly in my unsafe code.