functoriality of the category of elements

the new proof takes too much time:

    ./Elements.v:2804-2812: [Defined.] 2.149 secs (2.134u,0.014s)
    ./Elements.v:2787-2799: [reflexivity.] 0.31 secs (0.308u,0.002s)
    ./Elements.v:2767-2786: [apply~mor_equality.] 0.292 secs (0.291u,0.001s)
    ./Elements.v:2713-2732: [apply~mor_equality.] 0.208 secs (0.206u,0.001s)
    ./Elements.v:2733-2745: [reflexivity.] 0.18 secs (0.178u,0.001s)