Move some #includes around to match the Contributing guidelines.

The alias for __m128i is typically something like:

typedef long long __m128i __attribute__((__vector_size__(16), __may_alias__));

and the part that ends up not getting preserved is the __may_alias__
attribute specifier.

So, in order to preserve that, we can just use a wrapper struct, so the
data type itself isn't being passed through the template.

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.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.