Detect symmetries when generating code with FErari optimizations

FErari does not detect symmetries. FFC should detect symmetries (directly from UFL) and take advantage of symmetries when generating code. For example, the symmetric Laplacian comes out as:

A[1] = -0.5*G0_0_0 - 0.5*G0_1_0;
    A[5] = 0.5*G0_0_1;
    A[0] = -A[1] + 0.5*G0_0_1 + 0.5*G0_1_1;
    A[7] = 0.5*G0_1_0;
    A[6] = -A[7] - 0.5*G0_1_1;
    A[8] = 0.5*G0_1_1;
    A[2] = -A[8] - 0.5*G0_0_1;
    A[4] = 0.5*G0_0_0;
    A[3] = -A[4] - 0.5*G0_0_1;

A[1] should be equal to A[3] etc.

Thanks to Francis Russel for reporting.