Major improvements to robustness and speed

crew 0.2.0

• Use .args instead of ... in mirai::mirai().
• Use set.seed() instead of withr::local_seed() (#67, @shikokuchuo).
• Add a vignette to showcase integration with Shiny, co-authored with @dwoodie and with helpful suggestions by @shikokuchuo.
• Drop rlang::duplicate() (#72, @shikokuchuo).
• Do not poll dispatcher availability when starting router objects (@shikokuchuo).
• Implement a saturated() controller method to support targets.
• Add a worker_index column to controller summaries.
• When relaunching workers, prioritize those with unresolved tasks stuck at the NNG level (#75, @shikokuchuo).
• Always relaunch backlogged inactive workers (#79).
• Deprecate the auto_scale argument/field of controllers in favor of the scale argument of push(), pop(), and wait().
• Throttle auto-scaling with interval self$router$seconds_interval (#76).
• Remove clean() and instead terminate lost workers on launch.
• Fix examples.
• Add a collect argument to pop().