Don’t promote string channel values to functions. - #453

Yes. We still keep field() for shorthand? Is this what we want in d3 too? In that case should field check its argument x and return d=>d[x] iff x is a string (or, not a function)?

I only kept it to avoid repeating d => d[name] twice. It’s not exposed externally, and we only call it given a string name, so it doesn’t need to check that the argument is a string.