Simplified infrastructure and improved launcher plugin interface

crew 0.1.0

• Remove asyncdial from call to mirai::daemons().
• Explain vision in the launcher plugin vignette.
• Remove the superfluous clean() controller method.
• Clean up logic for scale() and launch() controller methods.
• Add a new inactive() launcher method to help with the above.
• Eliminate superfluous worker launches: clean up the controller logic, and check worker discoveries before worker connections (#51).
• Simplify the launcher plugin interface (#54).
• Commit to using the fast nanonext::.unresolved() (#49).
• Rename the callr launcher to the local process launcher (#56).
• Switch from SHA224 to SHA1 as the hash for random tokens and names (#59, @shikokuchuo).
• Open and confirm bus socket listeners asynchronously in the launcher abstract class (#58).
• Use condition variables to test the connection status of worker instances (#57, @shikokuchuo).
• Allow the dispatcher process to not exist so that the router will keep working if mirai implements the dispatcher another way (#60).
• Add a substitute argument to push() (#63).
• Remove dependency on custom NNG bus sockets, which significantly simplifies the internals, improves robustness, and smooths the integration with mirai (#61).
• Use processx instead of callr for the local process launcher (#64").
• Speed up crew_eval() by e.g. relying on nanonext::mclock() for the time.
• Add an empty() method to check if the controller or controller group is empty.
• Rely on mirai for globals cleanup, garbage collection, and global option resets (#65).
• Expose the package unloading capabilities of mirai (#65).
• Make monads classed environments and leave tibbles to the last minute in pop().
• Rename crew_wait() to crew_retry().
• Export crew_assert() and crew_random_name() to help launcher plugins.
• Rename the listen() method to start() in the router class.
• The poll() method now retries mirai::daemons() for robustness.
• crew_retry() gains a max_tries argument.
• Bug fix: stop setting router$daemons to NULL in poll() when the connection has a momentary problem.
• Simplify listening().