Traverser is a very simple functional interface whose shape matches
that of a Supplier, but with a contract specialized for the traversal
over a sequence of non-null items: each call to its next() method
returns another item of the sequence until exhausted, then keeps
returning null. The point of this type is the ability to implement
traversal over any kind of dataset or lazy sequence with minimum hassle:
often just by providing a one-liner lambda expression. This makes it
very easy to integrate into Jet's convenience APIs for cooperative
processors.
Traverser also supports some default methods that facilitate building
a simple transformation layer over the underlying sequence: map,
filter, and flatMap.