copy(A::Toeplitz) returns a regular Array

[BenjaminRemez]

Calling copy(A) for A::Toeplitz falls back to copy(AbstractArray), and then returns a dense, populated Array, instead of a new Toeplitz.

(This has an interesting side effect: it allows LinearAlgebra.det(A) to run successfully, since a copy during that routine causes a "densification" of A, whereas det(C::Circulant) fails, as Circulant implements a specialized copy method. This might we a separate wider issue to resolve.)