Needed maintenance before massiv-1.1.0.0 release

[lehins]

This is a checklist for me to do before making the next release:

• - Deprecate massiv-test in favor of sublibrary #145
• - Update fourmolu to latest version and fixup config
• - Make it possible to run fourmolu on the whole project: Fix CPP usage so fourmolu doesn't choke #120
• - Add fourmolu to CI #121
• - Fix performance issue: Massive memory use on a 3D stencil #142