Change sub behaviour to match getindex, possibly rename.

[simonbyrne]

Following #13612, we should change sub behaviour to match which, as I understand it, basically means renaming slice to sub (and deprecating slice).

There was some discussion in JuliaLang/LinearAlgebra.jl#255 of changing the name to view (which would conflict with ArrayViews), but if so we should make that decision soon.

cc: @andreasnoack @timholy @mbauman

[kmsquire]

👍 to changing to view. I think it's clearer about what's happening semantically.

[StefanKarpinski]

+1 to both