remove d3.functor

[gordonwoodhull]

We shouldn't modify the d3 namespace. Since d3 has dropped d3.functor we should either drop it or replace it.

I'm not sure of the rationale, but CHANGES.md recommends creating a constant() function instead

function constant(x) {
  return function() {
    return x;
  };
}

and replacing each call to d3.functor with

var fx = typeof x === "function" ? x : constant(x);

(We could create dc.constant() to make it available to our users.)

Perhaps it is marginally faster because the closure holding x has no code in it. Anyway, I think we should do this the idiomatic d3 way, even though it means breaking some user code.

Alternately we could implement dc.functor(), not use it internally, and immediately deprecate it, pointing to CHANGES.md for rationale. But it seems kind of silly to break user code and give a deprecated function as a quick fix.

[gordonwoodhull]

Fixed in 3.0 alpha 8