Convert Statement to use Ptr (#788)

Note this makes a few cases where the Statement was optional explicit (Block, If, Sequence). I do add a few CHECKs around where statements were optional and assumed but unverified.

I switch TypeCheckStmt to not take an optional Statement because I think it makes the call sites clearer in behavior. It's also a smaller change than the converse, because taking an optional Statement means the returned statement would also need to be optional. Arguably a wrapper for optional statements could be added, but this still seems cleaner to me, and there aren't that many cases of an optional statement.

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/interpreter/value.cpp

@@ -399,8 +399,14 @@ auto ValueEqual(const Value* v1, const Value* v2, SourceLocation loc) -> bool {
       return cast<BoolValue>(*v1).Val() == cast<BoolValue>(*v2).Val();
     case Value::Kind::PointerValue:
       return cast<PointerValue>(*v1).Val() == cast<PointerValue>(*v2).Val();
-    case Value::Kind::FunctionValue:
-      return cast<FunctionValue>(*v1).Body() == cast<FunctionValue>(*v2).Body();
+    case Value::Kind::FunctionValue: {
+      std::optional<Ptr<const Statement>> body1 =
+          cast<FunctionValue>(*v1).Body();
+      std::optional<Ptr<const Statement>> body2 =
+          cast<FunctionValue>(*v2).Body();
+      return body1.has_value() == body2.has_value() &&
+             (!body1.has_value() || *body1 == *body2);
+    }
     case Value::Kind::TupleValue:
       return FieldsValueEqual(cast<TupleValue>(*v1).Elements(),
                               cast<TupleValue>(*v2).Elements(), loc);