Avoid dynamic constant get on SET.symbols
Avoid dynamic constant get on SET.symbols

Dynamic constant lookups (ie. x::SOME_CONST) are uncommon and slightly
slow operations (not _that_ slow). We know that the ::SET constant won't
change (this is a private API and it is called only with our own
classes) so we can switch this to an attr_reader for fast access on the
call to delgate_to.

This also removes the unused `type_klass` attr_accessor.

yawboakye/rails,yahonda/rails,yawboakye/rails,rails/rails,yawboakye/rails,MSP-Greg/rails,Erol/rails,esparta/rails,esparta/rails,Erol/rails,georgeclaghorn/rails,esparta/rails,rails/rails,yahonda/rails,georgeclaghorn/rails,MSP-Greg/rails,rails/rails,georgeclaghorn/rails,georgeclaghorn/rails,BlakeWilliams/rails,yahonda/rails,Erol/rails,BlakeWilliams/rails,rails/rails,MSP-Greg/rails,esparta/rails,Erol/rails,BlakeWilliams/rails,yawboakye/rails,yahonda/rails,MSP-Greg/rails,BlakeWilliams/rails