Implement actualize for alt_var_length_view

[lums658]

This PR implements a function actualize that

• orders the underlying data of an alt_var_length_view according to the permuted subranges
• sets the subranges in the alt_var_length_view to delimit the new var-length element
• stores the size of each subrange in an index argument

After calling actualize, the alt_var_length_view will "look" exactly the same, i.e., iterating through it before and after will give exactly the same results (even though the underlying data range has been reordered).

Actually ordered (instead of virtually ordered) var-length data, along with an array of the corresponding sizes, will be needed for saving phase I data to a temporary array.

[sc-49496]

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TYPE: FEATURE
DESC: Implement actualize function that orders data underlying alt_var_length_view