NP canonization and truth table creation from Boolean equation

[aletempiac]

In constructors.hpp

• added a create_from_formula which creates a truth table from a Boolean equation

In npn.hpp

• added methods for NP and N canonization, and NP and N enumeration of functions

All of this is useful for technology mapping in mockturtle.

[lee30sonia]

Could you add the new functions in the documentation rst files and record them in the changelog before I merge?