Added MoleculeBuilderPostProcessingDissector.

Which is a supertrait for MoleculeDissector that allows post-processing
the value before returning it.
Add DissectorWithDefaultValue - exactly what it says on the tin.

This is a supertrait mainly for MoleculeBuilderPostProcessingDissector. The
latter's defaultValue method is now renamed to defaultWIP, to avoid a
collision.
RandomAdder: take DissectorWithDefaultValue instead of MoleculeBuilderDissector.
Rename the value parameter to wip in MoleculeBuilderPostProcessingDissector.dissectMB.
Turned several MoleculeBuilderDissectors into postprocessing ones.

And coarsened the type on their factory functions to not expose the
implementation class.
Added a synonym for MoleculeDissectorC[V] with DissectorWithDefaultValueC[V].
Updated RandomAdder's tests after the events of 31fdf7ed.
Optimized imports everywhere.