Replace standard list sort with better sort

[tanyongkiam]

The default list sort has quadratic behavior on sorted (or nearly sorted) lists because it picks the pivot from the head of the list.

This issue is about potentially replacing:

https://github.com/CakeML/cakeml/blob/master/basis/ListProgScript.sml#L405

with the "OCaml-inspired" mergesort available in HOL.

[tanyongkiam]

#1206 partially resolved this by defining a central "sort" that is shared across the codebase.

The next step is to implement a fast sort replacing this standardized sort.