Rules that are used only as auxiliary resolvers don't have an associated
origin node, even though they are produced from a specific location.
To enable tracking of correct origin to report errors to one can use
origin(no origin){} to override the default behaviour.
basetag() returns name of ConstraintRule without node id.
Check on whether Rule is principal are now done through basetag(), it's sufficient.
Add part of future analysis: CallerTableBuilder.
Step subclasses are meant to be immutable and stateless.
The state is now encapsulated in Effect subclasses, which can be combined.
Correspondingly, schedule now produces streams of effects.
Certain compromises have to be made: for one, it is assumed there is only one
project open and it is a MPSProject. Feedback is only reported to the messages
view and the view itself is not yet integrated into the main messages view.
Fedor Isakov