Documentation for method count-only

Documentation for method count-only, assembled from the following types:

role Iterator

From Iterator

(Iterator) method count-only

By default is not implemented, but expected implementation for types that do this role is:

method count-only(--> Int:D{ ... }

The method returns the number of elements the Iterator can still produce. Important: it's expected the Iterators that implement this method can produce that number without exhausting themselves. In other words, it's expected the user of the class will be able to still pull-one after calling this method, and eventually receive as many items as the return value of this method indicated.