SquareEye multiplication not specialized

[putianyi889]

Similar issue: #107

julia> A = rand(1000,1000);

julia> B=Eye(1000);

julia> @btime A*B;
  1.414 ms (3 allocations: 7.63 MiB)

julia> @btime copy(A)
  1.097 ms (3 allocations: 7.63 MiB)

Downstream issue:

julia> using ClassicalOrthogonalPolynomials, ToeplitzMatrices

julia> B=Toeplitz(1:∞,1:∞)
ℵ₀×ℵ₀ Toeplitz{Int64, InfiniteArrays.InfUnitRange{Int64}, InfiniteArrays.InfUnitRange{Int64}} with indices OneToInf()×OneToInf():
  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  …  
  2  1  2  3  4  5  6  7  8   9  10  11  12  13  14  15  16  17  18  19  20  21  22
  3  2  1  2  3  4  5  6  7   8   9  10  11  12  13  14  15  16  17  18  19  20  21
  4  3  2  1  2  3  4  5  6   7   8   9  10  11  12  13  14  15  16  17  18  19  20
  5  4  3  2  1  2  3  4  5   6   7   8   9  10  11  12  13  14  15  16  17  18  19
  6  5  4  3  2  1  2  3  4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  …
  7  6  5  4  3  2  1  2  3   4   5   6   7   8   9  10  11  12  13  14  15  16  17
  8  7  6  5  4  3  2  1  2   3   4   5   6   7   8   9  10  11  12  13  14  15  16
  9  8  7  6  5  4  3  2  1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
 10  9  8  7  6  5  4  3  2   1   2   3   4   5   6   7   8   9  10  11  12  13  14     
  ⋮              ⋮                ⋮                   ⋮                   ⋮          ⋱

julia> S=Legendre()
Legendre()

julia> S\(S*B) # freeze