The function `linesearch_objective` should not allocate a new `TemporaryUnivariateObjective` at every iteration

[benedict-96]

We should probably note create a new instance of TemporaryUnivariateObjective at every iteration, but store one in NewtonSolver. In this case we may also implement a new struct LinesearchObjective <: AbstractUnivariateObjective and replace TemporaryUnivariateObjective with it (as at the moment TemporaryUnivariateObjective is only used for line searches). In that way the terminology would also be more transparent, i.e. one would immediately know what LinesearchObjectives are for, whereas one has to look at the code at the moment to figure out where TemporaryUnivariateObjectives are used.