sortperm is unstable

[tpapp]

?sortperm says

The permutation is guaranteed to be stable even if the sorting algorithm is unstable, meaning that indices of equal elements appear in ascending order.

I am not sure if the algorithm in #773 is meant to be stable, but for StaticArrays v1.0.0,

julia> using StaticArrays

julia> v = [(9, 0.0), (9, 0.0), (9, 0.0), (9, 0.0),
       (3, 0.6000000000000001), (3, 0.6000000000000001)]
6-element Array{Tuple{Int64,Float64},1}:
 (9, 0.0)
 (9, 0.0)
 (9, 0.0)
 (9, 0.0)
 (3, 0.6000000000000001)
 (3, 0.6000000000000001)

julia> sortperm(v; by = last, rev = true)
6-element Array{Int64,1}:
 5
 6
 1
 2
 3
 4

julia> sortperm(SVector(v...); by = last, rev = true)
6-element SArray{Tuple{6},Int64,1,6} with indices SOneTo(6):
 5
 6
 2
 3
 1
 4

[c42f]

Yes we considered stability at length in #773 and I thought we had it correct. It's pretty subtle though, so I'm not completely surprised if we missed something.

@stev47 do you have time to look at this?