Stop allocating Actions on the Arena (#934)

This enables us to manage local variables in Carbon using C++ RAII.

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/interpreter/value.cpp

@@ -244,13 +244,7 @@ void Value::Print(llvm::raw_ostream& out) const {
       out << cast<VariableType>(*this).name();
       break;
     case Value::Kind::ContinuationValue: {
-      out << "{";
-      llvm::ListSeparator sep(" :: ");
-      for (Nonnull<const Action*> action :
-           cast<ContinuationValue>(*this).stack()) {
-        out << sep << *action;
-      }
-      out << "}";
+      out << cast<ContinuationValue>(*this).stack();
       break;
     }
     case Value::Kind::StringType:
@@ -264,6 +258,43 @@ void Value::Print(llvm::raw_ostream& out) const {
   }
 }
 
+ContinuationValue::StackFragment::~StackFragment() {
+  CHECK(reversed_todo_.empty())
+      << "All StackFragments must be empty before the Carbon program ends.";
+}
+
+void ContinuationValue::StackFragment::StoreReversed(
+    std::vector<std::unique_ptr<Action>> reversed_todo) {
+  CHECK(reversed_todo_.empty());
+  reversed_todo_ = std::move(reversed_todo);
+}
+
+void ContinuationValue::StackFragment::RestoreTo(
+    Stack<std::unique_ptr<Action>>& todo) {
+  while (!reversed_todo_.empty()) {
+    todo.Push(std::move(reversed_todo_.back()));
+    reversed_todo_.pop_back();
+  }
+}
+
+void ContinuationValue::StackFragment::Clear() {
+  // We destroy the underlying Actions explicitly to ensure they're
+  // destroyed in the correct order.
+  for (auto& action : reversed_todo_) {
+    action.reset();
+  }
+  reversed_todo_.clear();
+}
+
+void ContinuationValue::StackFragment::Print(llvm::raw_ostream& out) const {
+  out << "{";
+  llvm::ListSeparator sep(" :: ");
+  for (const std::unique_ptr<Action>& action : reversed_todo_) {
+    out << sep << *action;
+  }
+  out << "}";
+}
+
 auto TypeEqual(Nonnull<const Value*> t1, Nonnull<const Value*> t2) -> bool {
   if (t1->kind() != t2->kind()) {
     return false;