Implementing (static) labels for keys and values.
Fixing of hash code initialisation of transient sets.

Further adding invariant checks for hash code.
Adding a type bag for calculating the least upper bound of a collection.
Rewriting set to that single element operations use type bag.

Still to do: support batch operations.
Reactivating batch operations with reactive callbacks.
Fixing several typos after testing with PDB and Rascal tests.