Fix sort reduction support in backend

[vvergu]

Sort reductions of the form:

rules
  e : Expr --> v.

Are not working in the DynSem interpreter. The dispatch semantics should be:

Dispatching relation r on term t looks for a rule matching on constructorof(t)/arityof(t) and dispatches to that rule if it exists. Otherwise if a sort rule matching on sortof(t) exists it dispatches to that, or halt otherwise.

[vvergu]

Tightly dependent on #67.