This very much isn't a build configuration that we're going to ship,
but I want to be able to tell people that they can build it on their
own if they really want to see how terribly it performs :)

Just like before, you'll need to edit two lines in app/build.gradle to
define ENABLE_GENERIC=ON and actually enable armeabi-v7a if you want an
armeabi-v7a build. This commit just fixes some compilations errors that
crop up if you do so.

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.

This makes it easier to find the relevant functions.

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.