SI-7669 Fix exhaustivity warnings for recursive ADTs.

The pattern matcher's analysis was correctly finding
models under which the match in the enclosed test
could fail. But, when trying to render that model
as a counter example, it ran into an internal inconsistency
and gave up.

That inconsistency arose from VariableAssignment, which:

> turn the variable assignments into a tree
> the root is the scrutinee (x1), edges are labelled
> by the fields that are assigned a node is a variable
> example (which is later turned into a counter example)

In the process, it notes the unreachable case `V2 = NotHandled`,
which can only arise if `V1 = Op`, ie the scrutinee is `Op(NotHandled())`.
V2 is assosicated with the path `x1.arg`. The code then looked for
any variable assosicated with the prefix `x1` and registered that its
field `arg` was assosicated with this variable assignment.

However, the assignment for `V1 = NotHandled` (another missing case)
is also associated with the path `x1`. Registering this field makes
no sense here; we should only do that for `Op`.

This commit conditionally registers the fields based on the class
of `VariableAssignment#cls`. We no longer hit the inconsistency in
`VariableAssignment#allFieldAssignmentsLegal`.

This favourably changes the results of two existing tests.

I had to tweak the counter example pruning to avoid relying on
CounterExample.==, which is flaky in the light of Nil and List().
It is possible to have:

     A, B where A != B && A.coveredBy(B) && B.coveredBy(A)

Luckily it is straightforward to implement pruning entirely with
coveredBy.
Merge pull request #2737 from retronym/ticket/7669

SI-7669 Fix exhaustivity warnings for recursive ADTs.