This patch is a pure refactoring, which I've wanted to do for
some time.  The main payload is

* Remove the wc_insol field from WantedConstraints;
  instead put all the insolubles in wc_simple

* Remove inert_insols from InertCans
  Instead put all the insolubles in inert_irreds

* Add a cc_insol flag to CIrredCan, to record that
  the constraint is definitely insoluble

Reasons

* Quite a bit of code gets slightly simpler
* Fewer concepts to keep separate
* Insolubles don't happen at all in production code that is
  just being recompiled, so previously there was a lot of
  moving-about of empty sets

A couple of error messages acutally improved.