Spin the cron statement handling into submodule/crate

[stephanbuys]

Having the ability to interpret cron statements in other crates (without the timer/loop) would be useful. The 'next interval' could be emitted as a Duration to be used in other scenarios.

[mehcode]

This looks like its about done over at https://github.com/zslayton/cron. What's left is to rewrite the core of this to use that crate.