Replace BitUtils with C++20: RotateLeft/RotateRight

[JosJuice]

Now that we've flipped the C++20 switch, let's start making use of the nice new <bit> header.

I'm planning on handling this move away from BitUtils.h incrementally in a series of PRs. There may be a few functions remaining in BitUtils.h by the end that C++20 doesn't have any equivalents for.