Rename Label -> NonTerminal

Rename Generator -> ProductionRule

Adapt to latest bonsai

Tag ProductionRule's

Introduce TaggedGrammar, which tags non-terminals with the Tag of the
ProductionRule they came from + position.

Filter productions according to tags

During CEGIS, filter out programs with trivial operations such as (x == x)
(currently commented out)

Constant grammar detects constants in current FunDef

Commutative operations are now skewed to the right

Cegis individually checks candidate programs if they are few compared to total
programs.

Add grammars.transformers package, move some files there

Add sumToOrdered in SeqUtils

Add some API documentation

Empty grammar rules get printed

Warning when no functions chosen for synthesis

In CEGIS, individually test a few programs iff programs passing tests are
either very few, or much fewer than total programs.

In CEGIS, when we find a new counterexample, we filter the rest of remaining
programs with it before verifying them.