PR JuliaLang#52631 made `map` throw `MethodError` when called without an
iterator argument. That made `mapreduce` throw `MethodError` when
called without an iterator argument, too, something that was *not*
noticed back then. This change makes iteratorless `mapreduce` throw
before it can be called, instead of when it tries to call `map` (or
`Generator`) without passing it an iterator argument.

Now all of these functions should only have methods that take a
positive number of iterator arguments, which improves consistency:

1. `map`
2. `Iterators.map`
3. `foreach`
4. `reduce`
5. `foldl`
6. `foldr`
7. `mapreduce`
8. `mapfoldl`
9. `mapfoldr`
10. `Base.Generator`