Functional programming and TLS

crew 0.4.0

Highlights

• Use the TLS capabilities of nanonext and mirai.
• Add a map() method to the controller class for functional programming (#93).
• Fix bug in scale() which launched too many workers when active - demand was less than 0.
• Add crew_clean() to help clean up any lingering dispatchers and workers from previous sessions.

Other improvements

• Remove temporary controller summary workaround that ensured CRAN compatibility with targets 1.1.3.
• Add guardrails to make sure the controller is started for methods that require this. Part of this involved improving the error messages.
• Retry daemons() in daemons_info().