Add a Coproduct datatype

Coproduct is a sum type for different Functors with the same data type T.

e.g.
```java
Coproduct<stream,optional,Integer> nums = Coproduct.stream(1,2,3,OptionalKind.Instances.definitions());
int result = nums.map(i->i*2)
                 .foldUnsafe()
                 .foldLeft(Monoids.intSum);
//12
```