No characteristic function of MvNormal

[jaksle]

This is pretty confusing. The characteristic function of multivariate normal distribution cf(d::MvNormal, t::AbstractVector) is the simplest thing there is for it, and it seems it is not implemented! (The same for mgf.)

I could try adding it myself, but I see the current implementation of MvNormal is careful in handling general AbstractMatrix/AbstractVector as covariance matrix and mean vector, so that calculations are always efficient and I am not sure how to replicate that.