2nd derivative of Legendre polynomials

[MarcoFasondini]

 P = Legendre()
 x = axes(P,1)
 D = Derivative(x)
 D^2*Jacobi(0,0)

works but

 D^2*P

does not

[dlfivefifty]

This is actually an issue steming from InfiniteLinearAlgebra.jl: it doesn't know to leave D1 * D2 lazy for infinite-Toeplitz matrices:

julia> D1 = Ultraspherical(3/2) \ (D * P)
∞×∞ BandedMatrices.BandedMatrix{Float64,FillArrays.Ones{Float64,2,Tuple{Base.OneTo{Int64},InfiniteArrays.OneToInf{Int64}}},InfiniteArrays.OneToInf{Int64}} with indices OneToInf()×OneToInf():
  ⋅   1.0   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   …  
  ⋅    ⋅   1.0   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅      
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 ⋮                        ⋮                        ⋮                   ⋱  

julia> D2 = Ultraspherical(5/2) \ (D*Ultraspherical(3/2))
∞×∞ BandedMatrices.BandedMatrix{Float64,FillArrays.Fill{Float64,2,Tuple{Base.OneTo{Int64},InfiniteArrays.OneToInf{Int64}}},InfiniteArrays.OneToInf{Int64}} with indices OneToInf()×OneToInf():
  ⋅   3.0   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   …  
  ⋅    ⋅   3.0   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅      
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 ⋮                        ⋮                        ⋮                   ⋱