Simplify data.R

[dshemetov]

We can probably dispense with our manual delayed assign stuff and just use LazyData: TRUE as in here, since we no longer have R6 to worry about.

[brookslogan]

#520 is going to transform how we load data and will close this.

[brookslogan]

#520 is merged. Not the LazyData approach due to re-exporting from epidatasets, but does dispense with delayed_assign_with_unregister_awareness since we don't use R6 anymore, and is able to directly document with delayedAssign. (We've noted some issues with workflow for updating documentation but that's probably for a separate Issue.)