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...)

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.