Shrinking is not as good as we'd have hoped

We've implemented the shrinking from V1 on plans, but when I tried to enable some shrinking tests we find that most of the shrinking still doesn't work as well as we'd have hoped. This needs investigating. C.f. comments like this in `Constrained.Tests`:
```haskell
    -- TODO: figure out why this doesn't shrink
    testSpecNoShrink "reifyYucky" reifyYucky
```