Remove key template for iterators

[franmomu]

Q	A
Type	improvement
BC Break	no
Fixed issues

Summary

I think there is no need for specifying the key as template when using iterators, otherwise we (and users) always have to use: Iterator<key, User> instead of Iterator<User>.

I've used mixed as key because of Iterator::key definition. In the tests directory, all the occurrences I've found are using int.

[malarzm]

Thanks @franmomu!