Normals Forms supported

We include the basic normal forms for propositional formulas. They are negative normal form, disjunctive normal form, and conjunctive normal formal. All are available using a function or applying a theorem. See
more details in the NormalForms module.

First realease.

Equality (#3)

* [ Dec.agda ] eq function for decidibility

* [ Dec.agda ] eq and subs functions! :)

* cosmetics.