Support signal arrays assignment

[brech1]

We should add support for the assignment of whole signal arrays. Example:

template fc (width, height) {
    signal input weights[height][width];
    signal output out[height];
}

template network() {
    signal input in;
    signal output out;

    component l0 = fc(3, 5);

    signal input w0[5][3];
    signal input b0[5];

    l0.weights <== w0;
    ...
}