Add prox_evaluations to solver_specific in all solvers

[MohamedLaghdafHABIBOULLAH]

I suggest making prox_evaluations more explicit, for example:

prox_evals += solver.substats.iter

set_solver_specific!(stats, :prox_evals, prox_evals + 1)

what do you think @dpo @MaxenceGollier?

[dpo]

Good idea, but you have to make sure that set_solver_specific!() doesn't allocate.

[MohamedLaghdafHABIBOULLAH]

I guess it does not, because we've already used it in R2N