Yellow squiggly lines begone!
Done automatically on .cpp files through `run-clang-tidy`, with manual corrections to the mistakes.
If an import is directly used, but is technically unnecessary since it's recursively imported by something else, it is *not* removed.
The tool doesn't touch .h files, so I did some of them by hand while fixing errors due to old recursive imports.
Not everything is removed, but the cleanup should be substantial enough.
Because this done on Linux, code that isn't used on it is mostly untouched.
(Hopefully no open PR is depending on these imports...)

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.