Copy for CartesianIndices/LinearIndices need not materialize

[jishnub]

Currently,

julia> C = CartesianIndices((1:2, 1:2))
CartesianIndices((1:2, 1:2))

julia> copy(C)
2×2 Matrix{CartesianIndex{2}}:
 CartesianIndex(1, 1)  CartesianIndex(1, 2)
 CartesianIndex(2, 1)  CartesianIndex(2, 2)

However, seeing that a CartesianIndices is equivalent to an n-D range, there doesn't seem to be a need to materialize the result. This PR also ensures that copy(C) returns the same type as C.

After this PR:

julia> C = CartesianIndices((1:2, 1:2))
CartesianIndices((1:2, 1:2))

julia> copy(C)
CartesianIndices((1:2, 1:2))

Also, a similar change for LinearIndices is added.