Allow '0' to initialize R^d in function arguments and return

For instance, the following code is now allowed
``
// '0' argument interpreted as 0 in R^d
let f0: R^3->R, x -> x[2];
R x0 = f0(0);

let f1: R^3*R->R, (x,t) -> x[2]*t;
R x1 = f1(0, 1);

// '0' returned and interpreted as 0 in R^d
let f2: R->R^3*R, x->(0, 2*x);
R^3*R (x2,t2) = f2(2);

let f3: R->R^3, x->0;
R^3 x3 = f3(2.3);
``