_rvars analogs to _draws functions

[mjskay]

Once posterior hits CRAN it'll be time for putting together the tidybayes _rvars analogs to the _draws functions.

• [x] [spread/gather]_rvars family
  • [x] impl
  • [x] docs
  • [x] tests for spread
  • [x] tests for gather
  • [x] examples in vignettes
• [x] [add_][epred/predicted]_rvars family
  • [x] impl
  • [x] docs
  • [x] tests for epred
  • [x] tests for predicted
  • [x] examples in vignette
• [x] [add_]linpred_rvars
  • [x] impl
  • [x] docs
  • [x] tests
  • [x] examples in vignette
• [x] finish vignette
• [x] summarise_draws.grouped_df
  • [x] impl
  • [x] docs
  • [x] tests
  • [x] example in vignette
• [x] [un]nest_rvars
  • [x] impl
  • [x] docs
  • [x] tests
  • [x] example in vignette
• [x] point_interval
  • [x] impl
  • [x] docs
  • [x] tests
  • [x] example in vignette
• [x] compare_levels
  • [x] impl
  • [x] docs
  • [x] tests
  • [x] example in vignette