Use a lambda for tag's visit (#645)

Also shifts declaration order a little for [style](https://google.github.io/styleguide/cppguide.html#Declaration_Order); moving line_num down (data members after methods) and tag() down (factory functions before instance members).

executable_semantics/interpreter/value.h

@@ -163,8 +163,6 @@ struct ContinuationValue {
 };
 
 struct Value {
-  auto tag() const -> ValKind;
-
   // Constructors
 
   // Return a first-class continuation represented by the
@@ -216,6 +214,10 @@ struct Value {
   auto GetChoiceType() const -> const ChoiceType&;
   auto GetContinuationValue() const -> const ContinuationValue&;
 
+  inline auto tag() const -> ValKind {
+    return std::visit([](const auto& t) { return t.Kind; }, value);
+  }
+
  private:
   std::variant<IntValue, FunctionValue, PointerValue, BoolValue, StructValue,
                AlternativeValue, TupleValue, IntType, BoolType, TypeType,