Consolidate `Time::DayOfWeek` definition inside `Time`, adding documentation
and examples on its usage.

Refactor day question methods (ie `#monday?`) macro to use Enum's constants
and question methods, avoiding duplication of definitions.

This change does not introduce performance penalties by itself, but a
synthetic benchmark might present it otherwise:

    day_of_week.value == index: 626.33M (   1.6ns) (±10.72%)       fastest
         day_of_week.question?: 621.56M (  1.61ns) (±10.35%)  1.01× slower

    dow.value == index: 622.68M (  1.61ns) (± 9.82%)  1.00× slower
         dow.question?: 624.23M (   1.6ns) (±10.82%)       fastest

Benchmark code:

    require "benchmark"

    time = Time.new(2016, 2, 15)
    dow = time.day_of_week

    # benchmark against `day_of_week` being computed each time
    Benchmark.ips do |results|
      results.report("day_of_week.value == index:") { time.day_of_week.value == 1 }
      results.report("day_of_week.question?:") { time.day_of_week.monday? }
    end

    # benchmark against cached `day_of_week`
    Benchmark.ips do |results|
      results.report("dow.value == index:") { dow.value == 1 }
      results.report("dow.question?:") { dow.monday? }
    end