Improve API clarity of createEntity

[hmans]

createEntity should always only accept a single argument that must satisfy the world type. The current API allows for passing multiple partials, which is fun, but is a little too confusing (and requires us to loosen types a little too much.) For a component factory-centric approach, we can instead do this:

const entity = createEntity({
  ...position(0, 0),
  ...velocity(10, 0),
});

[hmans]

Coming in 0.11.