Introducing:

   1) Postponing the application of instances when there
      is a possibility of a given matching. With the addition
      of prioritizing equalities this fixes #5002 and #4981.

   2) Implemented caching of flattening in constraint
      simplification. This improves efficiency (fixes #5030)

   3) Simplified pushing of unsolved wanteds
      (now pushing only equalities) inside implications.