Should implement create_loop(autorun=TRUE)

[wch]

See here:

https://github.com/r-lib/later/blob/ae297fa9a51e7b162c26697f4ab9a97079020c59/R/later.R#L50-L54

This will eliminate the need for a callback that does the following:

• the callback schedules the private loop to run from the main loop.
• it reschedules itself.