Fix all -Wall and -Wextra compiler warnings (!80) · Merge requests · Proto / Darwin

Fixes a few other issues (returning pointers to temporaries, unused variables...).

Enables -Wall -Werror by default. On the CI, the warnings become errors. See here for the rationale not to use -Werror by default in CMake.

clang-tidy still has a lot to say, but that's for another time.

This will likely conflict with the trees MR.

Fix all -Wall and -Wextra compiler warnings