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

从 DelayedArray 构造函数开始，复现 issue 中的 IntegerList 示例，并将其与 DelayedArray(array(XX)) 进行比较。确定直接支持 List 所需的范围，并验证切片返回的是 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>