Accomplished using `run-clang-tidy` with `performance-move-const-arg,performance-unnecessary-value-param,modernize-pass-by-value`.

Changed arguments to const references, removed them where inappropriate (e.g. sink parameters). Same with std::move.

Manually reviewed each change to make sure that it makes sense, and do something more appropriate if possible.

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.