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.
Begins the transition to using fmt for string formatting where
applicable. Given fmt supports formatting std::string instances out of
the box, we can remove now-unnecessary calls to .c_str() and .data().
Note that this change does not touch the actual logging subsystem aside
from converting the final StringFromFormat call in the process over to
fmt::format. Given our logging system is heavily used throughout the
entire codebase, and converting that over will be quite a large change
by itself, this will be tackled near the end of the conversion process.
