testgen: minor: fixed a NPE
testgen: improved doc for option --concrete-sort
minor: added TODO comment for my experimental backend --unflatten-java
fixed a typo in kil.Attribute#BUILTIN_KEY
testgen: minor: avoided using hard-coded attribute name
testgen: implemented methods for computing common subsorts
testgen: taking into consideration that even the KLabel of a KItem is a constructor, the sort of this KItem may not be precise because the sorts of the free variables inside may become more specific