Separate Core into modules and write some unit tests

[andrew-johnson-4]

The compiler is too intertwined. Make a build path where we can create some stable interfaces and write unit tests for some minimal interactions.

• Types
  • Unification
  • Set Operations
  • Boolean Queries
  • Propositions and Corollaries
• Preprocessor
• Fragments

This is a good first issue for anyone who wants to work on compiler internals. The internals need to be refreshed and this is long overdue housekeeping.