Fix mutations in Axiom::Relation::Proxy#respond_to?
Kill mutations in Axiom::Relation::Proxy#method_missing

* Curiously this was done by removing the explicit test for
  #method_missing and allowing the specs for the public interface to
  cover it.
Fix mutations in Axiom::Relation::Proxy

* Move method declarations up to happen at module evaluation
  rather than at runtime when the module is included.
Fix mutations in Relation::Operation::Sorted
Change flay threshold