Fix Scan/Reduce/Collect Lambda Ambiguity

[benjchristensen]

This fixes the Java 8 lambda ambiguity between the overloads: #1881

This is a breaking change for anyone using scan, reduce, collect with an initial value.

There are two approaches to adapting.

If the initial value is the same type such as this:

stream.scan(1, Func2<Integer, Integer, Integer>)

then you can use startWith:

stream.startWith(1).scan(Func2<Integer, Integer, Integer>)

If it is a different type then a seed factory is needed:

stream.scan(Func0<List>, Func2< List, Integer, List >)

In Java 8 this would change from this:

stream.scan(new ArrayList(), (list, item) -> {
    list.add(item);
    return list;
});

to this:

stream.scan(() -> new ArrayList(), (list, item) -> {
    list.add(item);
    return list;
});

The same change happens in reduce and collect.

[benjchristensen]

cc @headinthebox for a review of this. This is cutting it close for 1.0 release but an important fix. Thanks @tommack for bringing this problem to our attention in #1831.

[benjchristensen]

Pulling the factory overloads from 1.0. Will take time to get right in 1.1.

[headinthebox]

@davgross can you update the docs to emphasize that mutable state is evil when doing aggregation?

[benjchristensen]

Fixed in #1884