This PR modifies the `induction`/`cases` syntax so that the `with`
clause does not need to be followed by any alternatives. This improves
friendliness of these tactics, since this lets them surface the names of
the missing alternatives:
```lean
example (n : Nat) : True := by
induction n with
/- ~~~~
alternative 'zero' has not been provided
alternative 'succ' has not been provided
-/
```
Related to issue #3555
