Create lookup for attribute names
Create lookup for attribute names

- Avoid using the to_s, split, last, to_sym chain. Improves performance
  another ~10%.

marshall-lee/grape-entity,ruby-grape/grape-entity,tyre/grape-entity,LeFnord/grape-entity,dan-corneanu/grape-entity,geniousli/grape-entity,calfzhou/grape-entity,cakejelly/grape-entity,intridea/grape-entity,danhodge/grape-entity,dblock/grape-entity,etehtsea/grape-entity,iangreenleaf/grape-entity