Handler mixin

[flash-gordon]

Add handler mixin

Now we can introduce and eliminate effects with a nicer API

class GetTimestamp
  include Dry::Effects[:current_time]

  def call
    # effect introduction
    { executed_at: current_time }
  end
end

include Dry::Effects::Handler[:current_time, as: :with_time]

def call
  op = GetTimestamp.new.freeze # op is immutable
  # effect elimination
  with_time.(Time.now) { op.() } # one time value
  with_time.(Time.now + 10) { op.() } # anothe time value
end

I'm thinking about replacing [] with () since it's unlikely we want to include many handlers at once. OTOH adding multiple effects makes sense so having [] may be reasonable for consistency.