feat: add Logics/Propositional/NaturalDeduction/*

[thomaskwaring]

Natural deduction for minimal logic

This PR, which depends on #89, adds the definitions and basic properties of natural deduction for propositional logic.

Key developments:

• Definition of Theory.Derivation, the type of derivation trees.
• Definitions of derivability and equivalence.
• Operations on derivations: weakening, substitution and cut.
• The preordering of theories by strength.