Switch interpreter Stacks to Ptr (#763)

This also eliminates the ctad wrapper for Stack: I think the leaning is to remove it. It felt worth keeping the constructor because constructing with a single element is a common use-case.

Adds a single-argument constructor for Scope because the `std::list<std::string>()` is common, and eliding it is consistent with what we've done for things like tuples.

I was considering a vector constructor due to the double-Push on line 1139, but thought the Push() semantics may mean that it's better not to provide.

executable_semantics/interpreter/stack.h

@@ -22,8 +22,6 @@ struct Stack {
   Stack() = default;
 
   // Creates an instance containing just `x`.
-  // TODO: consider removing this. It's somewhat unconventional, and the
-  // callsite readability is debatable.
   explicit Stack(T x) : Stack() { Push(std::move(x)); }
 
   // Pushes `x` onto the top of the stack.
@@ -46,7 +44,7 @@ struct Stack {
     CHECK(n >= 0) << "Negative pop count disallowed.";
     CHECK(static_cast<size_t>(n) <= elements.size())
         << "Can only pop as many elements as stack has.";
-    elements.resize(elements.size() - n);
+    elements.erase(elements.end() - n, elements.end());
   }
 
   // Returns the top element of the stack.
@@ -71,11 +69,6 @@ struct Stack {
   std::vector<T> elements;
 };
 
-// Explicitly enable CTAD to silence warnings.
-// TODO: consider removing this (and perhaps the associated constructor).
-template <typename T>
-Stack(T x) -> Stack<T>;
-
 }  // namespace Carbon
 
 #endif  // EXECUTABLE_SEMANTICS_INTERPRETER_CONS_LIST_H_