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.

We can make use of if constexpr for cases where booleans will always
statically be known at compile-time.

I had to rename Source/Common/Compiler.h because the VS build
confuses it with Externals/zstd/lib/common/compiler.h otherwise.

This is much better as prefixed double underscores are reserved for the
implementation when it comes to identifiers. Another reason its better,
is that, on Windows, where __forceinline is a compiler built-in, with
the previous define, header inclusion software that detects unnecessary
includes will erroneously flag usages of Compiler.h as unnecessary
(despite being necessary on other platforms). So we define a macro
that's used by Windows and other platforms to ensure this doesn't
happen.

Instead of globbing things under an ambiguous Common.h header, move
compiler-specifics over to Compiler.h. This gives us a dedicated home
for anything related to compilers that we want to make functional across
all compilers that we support.

This moves us a little closer to eliminating Common.h entirely.

This wouldn't be much of a data reader if it can't access the
read-only data pointer in read-only contexts. Especially if it
can get a writable equivalent in contexts that aren't read-only.

It's questionable to not return a reference to the instance being
assigned to. It's also quite misleading in terms of expected behavior
relative to everything else. This fixes it to make it consistent with
other classes.

Allows the default constructor to be defaulted and ensures the default
values are associated with the member variables directly.

Also corrects a prefixed underscore in the two parameter constructor.

These don't modify class state, so they can be const qualified.