More robust hessian-vector products

[gdalle]

Right now the function gradient_and_hessian_vector_product requires differeniating through a f!(y, x) with the inner backend. This is not possible for every backend, so we should have another trait layer that dispatches on whether or not the backend handles mutating function

[gdalle]

Necessary trait introduced in #55

[gdalle]

Also problematic is the fact that the test suite doesn't hit every line of that file yet

[gdalle]

Fixed by #83, closing in favor of more specific issues