Remove unused transform().
Move core algorithms to ufl/corealg/.

So far includes traversal, multifunction, and a new map_dag().
Refactoring.
Bugfixes. Tests pass again.
Implement ChangeToReferenceGeometry in terms of new non-recursive map_expr_dag algorithm instead of using the Transformer framework.
Simplify change to reference geometry algorithm by moving memoization
into a method decorator.
Implement change to reference grad algorithm in terms of new map_expr_dag.