Make ModelSchema.inheritance_column a class_attribute
Make ModelSchema.inheritance_column a class_attribute

Ultimately it's exactly the same semantic that was implemented
using `superclass`, except it avoid recursion and benefit from
method cache, so much faster the longer the inheritance chain.

The use of an alias_method chain is unfortunate, but I don't
see any alternative to cast the provided value to string.

Vasfed/rails,Stellenticket/rails,kddeisz/rails,MSP-Greg/rails,yahonda/rails,lcreid/rails,Stellenticket/rails,Envek/rails,kddeisz/rails,rails/rails,lcreid/rails,georgeclaghorn/rails,bogdanvlviv/rails,bogdanvlviv/rails,BlakeWilliams/rails,MSP-Greg/rails,esparta/rails,esparta/rails,bogdanvlviv/rails,Erol/rails,kmcphillips/rails,EmmaB/rails-1,joonyou/rails,Erol/rails,joonyou/rails,Edouard-chin/rails,kmcphillips/rails,Envek/rails,georgeclaghorn/rails,Edouard-chin/rails,Vasfed/rails,Erol/rails,MSP-Greg/rails,rails/rails,yawboakye/rails,kddeisz/rails,Vasfed/rails,Vasfed/rails,yawboakye/rails,joonyou/rails,yahonda/rails,MSP-Greg/rails,Erol/rails,BlakeWilliams/rails,yahonda/rails,yawboakye/rails,lcreid/rails,esparta/rails,kddeisz/rails,joonyou/rails,yahonda/rails,rails/rails,bogdanvlviv/rails,lcreid/rails,Stellenticket/rails,EmmaB/rails-1,yawboakye/rails,BlakeWilliams/rails,esparta/rails,Stellenticket/rails,Edouard-chin/rails,Envek/rails,kmcphillips/rails,Edouard-chin/rails,georgeclaghorn/rails,EmmaB/rails-1,georgeclaghorn/rails,Envek/rails,BlakeWilliams/rails,rails/rails,kmcphillips/rails