Returns a schedule map for a `cron-expr` whose parts correspond to
minute, hour, day-of-month, month, day-of-week. Each part is a comma
separated list of ranges or single numbers. Each range can be
specified either as '*' or as '<min>-<max>' and may be followed by a
'/<stepsize>' expression.

Example: '0,30 8-18/2 * * 1-5' results in schedule

{:minute      [{:from 10, :to 10, :step 1}
               {:from 30, :to 30, :step 1}],
 :hour        [{:from 8, :to 18, :step 2}],
 :day         [{:from 1, :to 31, :step 1}],
 :month       [{:from 1, :to 12, :step 1}],
 :day-of-week [{:from 1, :to 5, :step 1}]}