The unifier used to implement the `cases` tactic should not discard
equations of the form `x = t` and `t = x` using proof irrelanvance.
The new test demonstrates the issue. The unifier was reaching the
state
```
x : Conw Con.nil
|- x = Conw.nilw -> x = Conw.nilw
```
and discarding the equality instead of substituting `x`
because `x` and `Conw.nilw` are definionally equal due to
proof irrelevance.
@javra Do you have more complicated examples that were being
affected by this issue?
