BugFix: Ensure callbacks only run once per instance.

[CodeMeister]

Fixes: #1703

CallbacksObserver refactored to ignore an individual callback that has already been run on the given instance.

EXAMPLE

factory :user do
  name { "Jane" }

  trait :trait_a do
    trait_b
  end

  trait :trait_b do
    after(:build) { |user| user.name += " Doe" }
  end
end

user = build(:user, :trait_a, :trait_b)

BEFORE:
user.name #=> "Jane Doe Doe"

AFTER:
user.name #=> "Jane Doe"

See issue: #1703 for the full issue.