Add MonadFix?

Haskell has a class

```haskell
class Monad m => MonadFix m where
  mfix :: (a -> m a) -> m a
```

It should be possible to do something similar in PureScript. I'm not sure which of these types makes more sense:

```haskell
  mfix1 :: (Lazy a -> m a) -> m a
  mfix2 :: (Lazy a -> m (Lazy a)) -> m (Lazy a)
```