move Math::Model::{definitions} to Math::Model::AST::{definitions}
Fix errant definition of Dugout::Math::Model#ops

should return a list, not a list of lists.
Define ops in their own, clean namespace
refactor reifier to compile all ops together

We'll let the subtypes sort 'em out.
implement expression evaluator initial pass
refactor OpCompiler#initialize to inject dependencies more cleanly