Factor AllocationId out of Address (#916)

This lets us statically distinguish between code that works with arbitrary `Address`es and code that can only work with pointers to separately-allocated storage, and so we no longer need to worry about the latter code crashing at run-time (as `Heap::Deallocate` did) or silently doing the wrong thing (as `Heap::PrintAddress` did) if it's given the wrong kind of `Address`.

executable_semantics/interpreter/interpreter.h

@@ -44,7 +44,7 @@ class Interpreter {
                     SourceLocation source_loc) -> std::optional<Env>;
 
   // Support TypeChecker allocating values on the heap.
-  auto AllocateValue(Nonnull<const Value*> v) -> Address {
+  auto AllocateValue(Nonnull<const Value*> v) -> AllocationId {
     return heap_.AllocateValue(v);
   }