i_concat(obj) takes an iterable that returns
iterables, and chains together all inner values of iterables into
one iterator. Analogous to unlist(recursive=FALSE).
i_chain for iterators is analogous to c() on vectors. i_chain
constructs an iteror that returns elements from the first
argument until it is exhausted, then elements from the next
argument, and so on until all arguments have been exhausted.