Express stack updates using return values. (#747)

This enables the interpreter logic to express its intent more directly, especially in the common cases, and enables us to get rid of ValAction. It's also a step toward simplifying and encapsulating `state->stack`.

executable_semantics/interpreter/action.cpp

@@ -35,9 +35,6 @@ void Action::Print(llvm::raw_ostream& out) const {
     case Action::Kind::StatementAction:
       cast<StatementAction>(*this).Stmt()->PrintDepth(1, out);
       break;
-    case Action::Kind::ValAction:
-      out << *cast<ValAction>(*this).Val();
-      break;
   }
   out << "<" << pos << ">";
   if (results.size() > 0) {