CRAN release 0.12.0

• More minimal print methods for 'mirai' and 'miraiCluster'.
• Adds local_url() helper to construct a random inter-process communications URL for local daemons (thanks @noamross #90).
• daemon() argument 'autoexit' now accepts a signal value such as tools::SIGINT in order to raise it upon exit.
• daemon() now records the state of initial global environment objects (e.g. those created in .Rprofile) for cleanup purposes (thanks @noamross #91).
• Slightly more optimal as.promise() method for 'mirai'.
• Eliminates potential memory leaks along certain error paths.
• Requires nanonext >= 0.12.0.