ListIter class

• Handles most iteration and reification tasks for lists
• generation of lazy values
• Iterable interpolation
• flattening of Parcels (if flattening)
• removal of Nil values
• eagerness/infinity constraints

• Has a low-level array ($!rest) of things not-yet-reified

• When ListIter.reify($n) is called
• * start shifting things from $!rest
• * build a Parcel of reified objects after interpolation/flattening
• * callback any parent "container" to store/process the reified objects
• * create a new ListIter to handle any remaining lazy part in $!rest
• * return the Parcel and new ListIter to caller