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.

This prevents the IOS crypto code and keys from being spread over
the codebase. Things only have to be implemented once, and can be
used everywhere from the IOS code.

Additionally, since ES exposes some IOSC calls directly (DeleteObject
and Encrypt/Decrypt), we need this for proper emulation.

Currently, this only supports AES key objects.