More curry changes, including sometimes returning `functools.partial`:

1. Once again, `Curry` is a class and `curry` is a function.
2. `Curry`and `curry` may return a partial instance if there is a single
   argument remaining and no keywords.
3. `curried_object.call` was changed to `curried_object._call`.  As per
   usual convention, `_call` is an implementation detail that may occasionally
   be useful (as it is explicit and faster), but there are no guarantees
   that it will remain in future releases of `toolz`.