Improve lin_cons computation

[tmigot]

I'm still thinking there must be a better way here. If we know which constraints are linear (which we do), we can evaluate them (and therefore also their Jacobian) only once and store it. Every subsequent evaluation is a matrix-vector product followed by a vector addition (or subtraction). We don't need to call CUTEst again.

Originally posted by @dpo in #284 (comment)

We should also compute the linear constraint within the function

CUTEst.jl/src/julia_interface.jl

Line 372 in e89575d

function eval_lin_structure!(nlp::CUTEstModel)

so that cons_lin just do the matrix-vector product + vector.