Removed C/C++ examples.

The Prolog interface does the job very well and is more flexible.
The C/C++ examples were/would be just copies of tests anyway.
Formulas can be simplified before conversion to CNF.

For long formulas (in particular regressed formulas), this speeds up
CNF conversion considerably (Property 10 from 4s to 0.01s or so).

Also fixed [Knowledge|Belief]::Copy().