Improve performance by caching entity exposures
Improve performance by caching entity exposures

- Prevent constant rebuilding of Entity's @exposures class instance variable,
  improving serialization performance by 25%.

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