move dsl interpreter spec to a subdirectory in unit/core
rough in initial Interpreter::Node class
implement #on_association for Node Interpreter
implement #on_type for Node Interpreter
housekeeping
introduce node interpreter acceptance spec, remove unnecessary unit spec
fix #on_associations -> #on_association, add appropriate alias for #on_associations
rewrite #on_association unit spec

using the old-style spies to make the assertion a bit easier to manage
adjust incantation of spec
fix typo in spec