Add custom samplers, better collection, and better plotting to sinter

[Strilanc]

• Add sinter.Sampler and sinter.CompiledSampler classes
  • They can go anywhere a Decoder would go, but they are responsible for all parts of the sampling instead of only prediction
• Add a new default sampler perfectionist, which discards anything with detection events and predicts the observables are not flipped
• Improved layout of the progress printouts when collect is running
• Sinter decoders can now flag that they want to discard shots by adding an extra byte to the returned observable data, with 0 meaning keep and not-0 meaning discard
• Change how sinter collect distributes work
  • Workers are now distributed as widely as possible, instead of all on one task
  • Workers are now never switched between tasks until their current task is done
• Add sinter plot --point_label_func argument for drawing text next to data points
• Augment sinter plot --group_func to support dictionaries with special keys controlling precise grouping behaviors
  • If group_func returns a dict with a "color" key, all items with the same "color" value are drawn with the same color
  • If group_func returns a dict with a "linestyle" key, all items with the same "linestyle" value are drawn with the same linestyle
  • If group_func returns a dict with a "marker" key, all items with the same "marker" value are drawn with the same marker
  • If group_func returns a dict with a "label" key, this forces the label shown in the legend
  • If group_func returns a dict with an "order" key, this takes priority for ordering the legend
• sinter collect --processes is no longer required (defaults to "auto")
• sinter plot --show is no longer required (defaults to showing, unless --out is specified, unless --show is specified)
• Group some of sinter's code into private subpackages
• Show traditional error bars instead of a filled region for high/low fit when only one data point is present
• Add sinter plot --preprocess_stats_func
• Add sinter.TaskStats.with_edits
• Add safety error when adding stats that have equal strong ids but differing identifying information (json_metadata or decoder)

Some of the sampler design is adapted from @inmzhang's design in #735

Fixes #774

Fixes #682

Fixes #392