Restructure cabal.project files.

[wenkokke]

This PR restructures cabal.project into cabal.project.release and cabal.project.debug so that we can switch between release/debug builds from the command line rather than by editing a file, to avoid the need to git stash/apply when switching branches, and to enable an entirely programmatic git bisect for benchmarks. It also symlinks cabal.project.debug to cabal.project as the default configuration.