In-place merge sort for slices

[tahina-pro]

This PR reorganizes the implementations of in-place merge sort:

• It renames Pulse.Lib.ArraySwap into Pulse.Lib.Swap.Array (cf. FStarLang/steel#12)
• It introduces Pulse.Lib.Swap.Slice to swap the contents of two adjacent regions within a slice
• It renames Pulse.Lib.Array.MergeSort into Pulse.Lib.Sort.Merge.Array (cf. #245 )
• It introduces Pulse.Lib.Sort.Merge.Slice to sort the contents of a slice using in-place merge sort. (Like the array implementation, it is optimized for memory consumption rather than speed.)

[tahina-pro]

This PR also marks the element type argument of Pulse.Lib.ArrayPtr.ptr and Pulse.Lib.Slice.slice strictly positive. This is needed to use slices in recursive type definitions.