Reduce allocations in set_callbacks
Reduce allocations in set_callbacks

This method is the source of many useless allocations:

```
allocated objects by location
-----------------------------------
     39568  ~/gems/rails-25a5b296a310/activesupport/lib/active_support/callbacks.rb:846

allocated memory by location
-----------------------------------
  17.11 MB  ~/gems/rails-25a5b296a310/activesupport/lib/active_support/callbacks.rb:846

retained memory by location
-----------------------------------
   1.03 MB  ~/gems/rails-25a5b296a310/activesupport/lib/active_support/callbacks.rb:846
```

kddeisz/rails,esparta/rails,Vasfed/rails,EmmaB/rails-1,yalab/rails,tgxworld/rails,rafaelfranca/omg-rails,joonyou/rails,Erol/rails,Edouard-chin/rails,schuetzm/rails,kddeisz/rails,joonyou/rails,repinel/rails,eileencodes/rails,Edouard-chin/rails,georgeclaghorn/rails,lcreid/rails,lcreid/rails,yahonda/rails,Erol/rails,yalab/rails,yahonda/rails,yalab/rails,Vasfed/rails,BlakeWilliams/rails,BlakeWilliams/rails,schuetzm/rails,betesh/rails,starknx/rails,yawboakye/rails,kmcphillips/rails,starknx/rails,tgxworld/rails,kddeisz/rails,tjschuck/rails,yahonda/rails,MSP-Greg/rails,rafaelfranca/omg-rails,lcreid/rails,MSP-Greg/rails,vipulnsward/rails,kddeisz/rails,rails/rails,eileencodes/rails,yahonda/rails,Erol/rails,palkan/rails,yawboakye/rails,bogdanvlviv/rails,yalab/rails,EmmaB/rails-1,betesh/rails,Envek/rails,rails/rails,yawboakye/rails,BlakeWilliams/rails,vipulnsward/rails,Stellenticket/rails,tjschuck/rails,bogdanvlviv/rails,yawboakye/rails,palkan/rails,MSP-Greg/rails,vipulnsward/rails,joonyou/rails,betesh/rails,MSP-Greg/rails,lcreid/rails,kmcphillips/rails,joonyou/rails,tgxworld/rails,tjschuck/rails,Stellenticket/rails,Envek/rails,georgeclaghorn/rails,palkan/rails,BlakeWilliams/rails,georgeclaghorn/rails,esparta/rails,palkan/rails,rails/rails,vipulnsward/rails,eileencodes/rails,eileencodes/rails,georgeclaghorn/rails,bogdanvlviv/rails,Vasfed/rails,bogdanvlviv/rails,repinel/rails,Vasfed/rails,Edouard-chin/rails,Envek/rails,Edouard-chin/rails,betesh/rails,starknx/rails,schuetzm/rails,rails/rails,EmmaB/rails-1,kmcphillips/rails,schuetzm/rails,rafaelfranca/omg-rails,tgxworld/rails,esparta/rails,kmcphillips/rails,Stellenticket/rails,Envek/rails,esparta/rails,repinel/rails,tjschuck/rails,Erol/rails,repinel/rails,Stellenticket/rails