User-facing objective/gradient API

[klamike]

e.g. dual_objective(model, y, p) and dual_objective_and_gradient(model, y, p) that takes care of the y shapes

Need to consider batching and CUDA compat here.

[klamike]

Most of this is in place thanks to #12. I'd still like to define some special batch dispatches for X::AbstractMatrix so people can do dual_objective(X) directly instead of dual_objective.(eachcol(X))