allow wraparound indices for matrices as we do for lists


    i11 : {1,2,3,4}_-1

    o11 = 4

    i12 : {1,2,3,4}#-1

    o12 = 4


    i6 : R = QQ[a..d]

    o6 = R

    o6 : PolynomialRing

    i7 : f = vars R

    o7 = | a b c d |

		 1       4
    o7 : Matrix R  <--- R

    i8 : f_2

    o8 = | c |

	  1
    o8 : R

    i9 : f_-2
    stdio:9:2:(2):[3]: subfreemodule: index out of bounds

    i10 : f_{-1,0,1}
    stdio:10:2:(2):[3]: subfreemodule: index out of bounds
