Remove dependency on d3

There are a few places where D3 is [used but not needed](https://github.com/Esri/cedar/blob/837bf81af6fdb1ccba41fa740383992a83864a67/src/cedar.js#L995). Need to make sure we only use d3 where we need to. Ideally, this library would depend on just vega, or once Cedar uses Rollup/ES2015, perhaps bring in only the bits of d3 (v4) that are needed via tree shaking.