avoid computing jd for checking Date#julian?

* weird case of JI/JVM opti? Instant#isAfter() is `a.getMillis() > b.getMillis()`
 isAfter()
  Date#julian?     1.124 µs/i ± 0.024 ( 2.1%) <=>    889477 ips
  Date.new         2.430 µs/i ± 0.043 ( 1.8%) <=>    411488 ips
  Date.new.julian? 6.830 µs/i ± 0.269 ( 3.9%) <=>    146417 ips

 direct comparison
  Date#julian?     1.160 µs/i ± 0.038 ( 3.3%) <=>    862193 ips
  Date.new         2.318 µs/i ± 0.036 ( 1.6%) <=>    431411 ips
  Date.new.julian? 3.961 µs/i ± 0.060 ( 1.5%) <=>    252462 ips
use getDayOfWeek() for Date#wday
remove extra line between methods and modifiers (private, once, aliases, ...)

... so one can see easily they belong together.
* also remove an old "require 'enumerator'"
use Date.julian_leap?/Date.gregorian_leap? for Date#leap?