possble DelayedArray constructor over `IntegerList` (or other `List`)

Start at the DelayedArray constructor and reproduce the IntegerList example from the issue, comparing it with DelayedArray(array(XX)). Determine the scope needed for direct List support and verify that slicing returns an IntegerList rather than an ordinary list.

Hi @hpages ,

In the VariantAnnotation package, the CollapsedVCF (or ExpandedVCF) are saving the data entries in IntegerList / CharacterList.... And in the development of VCFArray, we are trying to represent these data entries as DelayedArray instances. Now we are converting the data entries into array to add dimension, and then use the DelayedArray constructor over the array. Is it possible to have the DelayedArray constructor directly work on List object? so that the internal data saving are still using a more efficient way in List structure than the ordinary list? @mtmorgan

 > XX <- IntegerList(c(list(rep(NA, 2)), list(rep(NA, 2)), list(rep(NA, 2)), list(rep(NA, 2))))
 > XX
 IntegerList of length 4
 [[1]] <NA> <NA>
 [[2]] <NA> <NA>
 [[3]] <NA> <NA>
 [[4]] <NA> <NA>

 > DelayedArray(array(XX))
 <4> DelayedArray object of type "list":          ## the type is ordinary list. 
    [1]    [2]    [3]    [4]
 NA, NA NA, NA NA, NA NA, NA

!> DelayedArray(array(XX))[1:2]  
 [[1]]
 [1] NA NA

 [[2]]
 [1] NA NA

## What we want is something like: 

!> DelayedArray(XX)
 <4> DelayedArray object of type "IntegerList":      
    [1]    [2]    [3]    [4]
 NA, NA NA, NA NA, NA NA, NA

!> DelayedArray(XX)[1:2]  
 IntegerList of length 2
 [[1]] <NA> <NA>
 [[2]] <NA> <NA>