Add the `updatePos` function as part of the `Parser` type.

The addition of the `updatePos` function as `u` in the `Parser` type
allows minimal management of the position updates in parsing code. It
also allows correct position updates in the `Parser` Alternative and
Monad.
Introduce `PosUpd a p` as an alias for positioning updates function.
Rename `Predicate a` to `Pred a`. Only internal alias.
Minor update to comment strings
Update README with Changelog