iterator: Add `IteratorUtil::flat_map_` method

flat_map_ produces an iterator that maps each element to an iterator,
and yields the elements of the produced iterators.

This is the monadic bind :: M a -> (a -> M b) -> M b  for iterators.

Named just like the vec method, but with a trailing underline until the
method resolution bug is resolved.

We discussed the name chain_map, but I decided to go with flat_map_ for consistency with vec.

Since it.map(f).flatten()  would be the same as it.flat_map(f), we could choose
to just implement a flatten method instead. Either way the possibilities are the same but flat_map is more convenient.