IntStream.sum

[Remi Forax]

IntStream.sum have just been changed to return a long instead of an int 
and I don't think it's a good idea.

There is 3 ways to deal with overflows:
   1) do nothing, the Java default semantics
   2) throw an exception if overflown (using Integer.addExact by example)
   3) use BigInteger
all other ways are just half baked solution.

This one is very bad because usually users will use it that way,
   int sum = (int)intStream.sum();
so it doesn't solve anything, just make the semantics awkward.

sum() is just a shortcut for a reduce, so it should stick with the 
default Java semantics,
if users want longs, they can use reduce with Long::add or Long::addExact.

Rémi