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.

Given bit conversions between types are quite common in emulation
(particularly when it comes to floating-point among other things) it
makes sense to provide a utility function that keeps all the boilerplate
contained; especially considering it makes it harder to accidentally
misuse std::memcpy (such as accidentally transposing arguments, etc).

Another benefit of this function is that it doesn't require separating
declarations from assignments, allowing variables to be declared const.
This makes the scenario of of uninitialized variables being used less
likely to occur.

This one verifies bitmasks where low bits are set to 1 (hence the name).
Any stray 0 among the lower ones or any stray 1 among the higher zeros
renders the mask invalid.

The edge cases of all zeros and all ones are considered valid masks.

It uses an efficient implementation. It's the counterpart of
https://graphics.stanford.edu/~seander/bithacks.html#DetermineIfPowerOf2

Fixes warnings like:

```
dolphin/Source/UnitTests/Common/BitUtilsTest.cpp:5:
../Externals/gtest/googletest/include/gtest/gtest.h:1392:11: warning: comparison of integers of different signs: 'const unsigned long' and 'const int' [-Wsign-compare]
  if (lhs == rhs) {
      ~~~ ^  ~~~
../Externals/gtest/googletest/include/gtest/gtest.h:1421:12: note: in instantiation of function template specialization 'testing::internal::CmpHelperEQ<unsigned long, int>' requested here
    return CmpHelperEQ(lhs_expression, rhs_expression, lhs, rhs);
           ^
dolphin/Source/UnitTests/Common/BitUtilsTest.cpp:12:3: note: in instantiation of function template specialization 'testing::internal::EqHelper<false>::Compare<unsigned long, int>' requested here
  EXPECT_EQ(Common::BitSize<s8>(),  8);
  ^
../Externals/gtest/googletest/include/gtest/gtest.h:1924:63: note: expanded from macro 'EXPECT_EQ'
                      EqHelper<GTEST_IS_NULL_LITERAL_(val1)>::Compare, \
```

This attempts to make some bit arithmetic more self-documenting and also
make it easier during review to identify potential off-by-one errors by
making it possible to just specify which bits are being extracted.

Functions both support the case where bits being extracted can vary and
fixed bit extraction. In the case the bits are fixed, compile-time asserts
are present to prevent accidental API usage at compile-time.

e.g. Instead of shifting and masking to get bits 10 to 15,
Common::ExtractBits<10, 15>(value) can just be done instead.