Finished refactoring according to feedback.

– Type checking classes reduced to TypeCheckerVisitor and
TypeEnvironment. No ErrorCollection, LabelCollection classes anymore.
– TypeEnvironment knows nothing about the way errors are collected,
just returns an error when necessary.
– Errors are no longer categorised by Class, just put in a list.
– Renamed some classes (removed QL prefix).
– Refactored incorrectly formatted file JDocs.