Gets this out of the global namespace.

This isn't used anywhere and not really a generic utility, so we can get
rid of it.

This also lets us remove MathUtil.cpp, since this was the only thing
within that file.

With the upgrade to C++20, std::countl_zero and std::countr_zero can replace these home-spun implementations from the BitUtil.h library.

It's unused in the case that T is unsigned and dest is signed (e.g. SaturatingCast<int, size_t> which appears SetIsoPaths in MainSettings.cpp)

Mostly everything else was already marked as constexpr, and the only
function IntLog2 calls (CountLeadingZeros) is already constexpr.

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.

`std::abs(x - y)` where x and y are unsigned integers fails to compile
with an "call of overloaded 'abs(unsigned int)' is ambiguous" error
on GCC, and even if it did compile, that expression still wouldn't
give the correct result since `x - y` is unsigned.