Closes hedgehogqa#448. Suppose we have an Action list

    A 1 -> Var 0
    B (Var 0) -> ...

Then when we shrink A, we would previously get the list

    A 0 -> Var 1
    B (Var 0) -> ...

And then we'd drop B from this list, because `Var 0` no longer exists.
Now shrinking will give

    A 0 -> Var 0
    B (Var 0) -> ...

which is fine.

This means we now generate Vars even for actions whose `Require` fails.

…o tripping-m