allow calculating all distribution functions at once in stat / stat_dist

[mjskay]

then stick them into mappings users can use, like pdf and cdf. Would allow experimentation with even weirder custom geoms (like the Correll-style violins with faded tails)

• stat_dist_...
• stat_sample_...
• fix the appearance of some NAs in the CDF calculation in stat_dist. See here: continuous fill for geom_halfeyeh() tidybayes#136 (comment) this is due to interval data also being processed in the same geom; the fix is to set na.translate = FALSE on the fill scale. Added to examples
• add a function for making intervals from cdfs (maybe cut_cdf_qi?)
  • re-export from tidybayes
  • add example to slabinterval vignette
• add docs
• add example to vignette