Refactor CEGIS

NondeterministicProgram encapsulates all handling of programs
NonDeterministicProgram can be an object
Fix GrowableIterable and its usage in CEGIS
Don't include productions that lead to empty nonterminals
Remove activeBs (unused)
Move validatePrograms within main CEGIS loop
Remove usage of Streams
Fix usage of Either
Fix, comment and move things around
Add a check for redundat programs in (deactivated for now)