Implement farray +* operators

This will work for both Function:

a * 2 = [a, a]
a + a = [a, a]

and farray types:

xs = exprvars('x', 3)
ys = exprvars('y', 3)
xs * 2 = [x[0], x[1], x[2], x[0], x[1], x[2]]
xs + ys = [x[0], x[1], x[2], y[0], y[1], y[2]]

It will always flatten everything, so if you want shaped data,
use the `farray.reshape` method.