Rename Ptr<T> to Nonnull<T*> (#832)

Note that ptr.h also includes an enable_if change, to help avoid https://bugs.llvm.org/show_bug.cgi?id=51881

executable_semantics/ast/declaration.cpp

@@ -20,7 +20,7 @@ void Declaration::Print(llvm::raw_ostream& out) const {
       const ClassDefinition& class_def =
           cast<ClassDeclaration>(*this).Definition();
       out << "class " << class_def.name << " {\n";
-      for (Ptr<Member> m : class_def.members) {
+      for (Nonnull<Member*> m : class_def.members) {
         out << *m;
       }
       out << "}\n";