This formats all copyright comments according to SPDX formatting guidelines.
Additionally, this resolves the remaining GPLv2 only licensed files by relicensing them to GPLv2.0-or-later.

Now that clang-format makes [[nodiscard]] attributes format sensibly, we
can apply them to several functions within the common library to allow
the compiler to complain about any misuses of the functions.

Makes it explicit that the header is intended for iterator-based
algorithms that can ideally operate on any type.

Makes the header more general for other potential algorithms in the
future. While we're at it, include a missing <functional> include to
satisfy the use of std::less.