This refactoring was triggered by Trac #10675.

We were using 'improveClsFD' (previously called 'checkClsFD') for
both
  * Improvement: improving a constraint against top-level instances
  * Consistency: checking when two top-level instances are
    consistent

Using the same code for both seemed attractive at the time, but
it's just too complicated.  So I've split it:
 * Improvement: improveClsFD
 * Consistency: checkFunDeps

Much clearer now!