ggdist: Visualizations of distributions and uncertainty

New features:

• Added "weave" and "swarm" layouts for dots geoms (#64). These provide alternative layouts that keep datapoints in their actual positions on the data axis. The "weave" layout maintains rows but not columns and works well for quantile dotplots; the "swarm" layout uses the "compactswarm" method from beeswarm::beeswarm() (courtesy James Trimble) and works well on sample data. See the dotplot section of vignette("slabinterval") for comparisons.
• Allow the use of unit() to specify bin widths manually for dots geoms and stats, which can be helpful when you need dotplots across facets to have the same bin width (#53).

New documentation:

• Add example of lineribbon gradients using fill_ramp in vignette("lineribbon").
• Add example of Tukey-like pencils in vignette("slabinterval").
• Add example of two slab used together (densities and dotplots to make "rain clouds") in vignette("slabinterval").

Bug fixes:

• Fix issues with ggplot2 3.3.4 (#72) and vdiffr 1.0.
• Handle interactions between alpha and fill/color properly when not set by user (#62).
• Use step function for all ECDFs, which should also fix constant CDFs (#55).
• Move fda to suggests as it brings in a large number of dependencies and is rarely used.
• Use trimmed density for mode estimation (#57).