:: [a] -> [[a]] package:leancheck

Takes a list of values xs and transform it into tiers on which each tier is occupied by a single element from xs.
toTiers [x, y, z, ...]  =  [[x], [y], [z], ...]
To convert back to a list, just concat.
Classify values using their Eq instance.
> classify [1,2,3,1,2,1]
[[1,1,1],[2,2],[3]]
(cf. classifyBy, classifyOn)