Naming of "ireturn"

With the AMP update, `ireturn` is somewhat outdated. I propose a new naming scheme:

```
class IxPointed where ipoint :: ..

ipure :: IxApplicative m => a -> m i i a
ipure = ipoint

ireturn :: IxMonad m => a -> m i i a
ireturn = ipoint
```

This is of course a breaking change for any existing instances.
