§ Module system for separate compilation

• https://github.com/leanprover/lean4/issues/416
• https://www.cs.utah.edu/plt/publications/macromod.pdf
• https://raw.githubusercontent.com/alhassy/next-700-module-systems/master/phd-defence.pdf
• https://raw.githubusercontent.com/alhassy/next-700-module-systems/master/thesis.pdf