Improve CNF generation ergonomics

[Jannis Harder]

Generating CNF formulas when modeling problems should be improved by:

• Adding .pos() and .neg() and .lit(polarity: bool) to Var
• Adding a .new_var() to CnfFormula and to Solver that return the next unused variable (and mark it as used).
• Also add .new_lit(), .new_vars(len: usize) and .new_lits(len: usize) to CnfFormula and to Solver.

This needs to be added to the manual and the examples should also be updated where it makes them simpler.