Iterator#next should have return type restriction

[straight-shoota]

The module Iterator(T) revolves around the abstract def #next which is expected to return T or a stop indicator (Iterator::Stop). This expectation is currently not expressed in a type restriction. But it should be: Iterator(T)#next : T | Iterator::Stop.

Without this type restriction, an Iterator implementation can return any type in its #next method, which is prone to cause issues further down the line (see #10857 (comment) for an example). Adding this type restriction makes it easy to spot any incorrect implemtnations.

Unfortunately, this is currently not possible without introducing a major breaking change, because all abstract def implementations would be forced to specify a return type restriction as well. And most Iterator implementations don't do that.

#10904 proposes a compiler change as an alternative to introduce this return type restriction without a breaking change.