Accept IndexStyle in diagind, defaulting to IndexLinear

[jishnub]

Partly revert and redesign #52115, with diagind now accepting an optional IndexStyle, which is IndexLinear by default. This should address the breakages reported in that PR. After this,

julia> D = Diagonal(1:4)
4×4 Diagonal{Int64, UnitRange{Int64}}:
 1  ⋅  ⋅  ⋅
 ⋅  2  ⋅  ⋅
 ⋅  ⋅  3  ⋅
 ⋅  ⋅  ⋅  4

julia> diagind(D)
1:5:16

julia> diagind(D, IndexCartesian())
StepRangeLen(CartesianIndex(1, 1), CartesianIndex(1, 1), 4)

[dkarrasch]

Are these all internal usages of diagind? I assume so.

[jishnub]

I didn't think of the diagind change as internal, as other matrix types such as BandedMatrices would also benefit from a Cartesian range

[dkarrasch]

I meant the callsites, not the method definition itself. Like, have you optimized all "internal" calls to diagind? But since you have done that already in very different places, I was assuming that.

[jishnub]

Oh alright, yes, I've updated the internal calls. Not all of them at the moment, as this only makes a difference if the rest of the operations are also O(N). As a matter of style, though, we might decide to change it everywhere. If we decide this, I'll make further changes that aren't strictly necessary for performance.