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

從 DelayedArray 建構函式開始，重現 issue 中的 IntegerList 範例，並將其與 DelayedArray(array(XX)) 進行比較。確定直接支援 List 所需的範圍，並驗證切片回傳的是 IntegerList，而不是普通 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>