Adds shift_right and shift_left functions to algorythems as specified in P0769R2.
In this implementation, I use std::advance to shift the iterators instead of the proposed sample algorithm. This seems to work fine and be much faster, but there may be some reason not to do this which I am unaware of.
The document suggests that _only_ shift_left is a constexpr while all other functions/overloads are not. I think this is a typo, but I can update if needed (edit: emailed to inquire about this).