Enable -Wshadow and fix warnings. (#3909)

Enable -Wshadow and fix warnings. (#3909)

Fixes all -Wshadow warnings and enables the -Wshadow compile flag globally.

Co-authored-by: Clark Barrett <barrett@cs.stanford.edu>
Co-authored-by: Andres Noetzli <andres.noetzli@gmail.com>
Co-authored-by: Aina Niemetz <aina.niemetz@gmail.com>
Co-authored-by: Alex Ozdemir <aozdemir@hmc.edu>
Co-authored-by: makaimann <makaim@stanford.edu>
Co-authored-by: yoni206 <yoni206@users.noreply.github.com>
Co-authored-by: Andrew Reynolds <andrew.j.reynolds@gmail.com>
Co-authored-by: AleksandarZeljic <zeljic@stanford.edu>
Co-authored-by: Caleb Donovick <cdonovick@users.noreply.github.com>
Co-authored-by: Amalee <amaleewilson@gmail.com>
Co-authored-by: Scott Kovach <dskovach@gmail.com>
Co-authored-by: ntsis <nekuna@gmail.com>