refactor: make reference's interface hidden friends

Addresses:
- https://github.com/mpusz/units/pull/261#discussion_r597135187
- https://github.com/mpusz/units/pull/261#discussion_r597152408

src/core/include/units/quantity.h

@@ -404,12 +404,6 @@ template<Quantity Q1, QuantityEquivalentTo<Q1> Q2>
   return ret(ret(lhs).count() - ret(rhs).count());
 }
 
-template<QuantityValue Rep, typename D, typename U>
-[[nodiscard]] constexpr Quantity auto operator*(const Rep& lhs, reference<D, U>)
-{
-  return quantity<D, U, Rep>(lhs);
-}
-
 template<Quantity Q1, Quantity Q2>
   requires quantity_value_for_<std::multiplies<>, typename Q1::rep, typename Q2::rep>
 [[nodiscard]] constexpr Quantity auto operator*(const Q1& lhs, const Q2& rhs)
@@ -417,12 +411,6 @@ template<Quantity Q1, Quantity Q2>
   return detail::make_quantity<Q1::reference * Q2::reference>(lhs.count() * rhs.count());
 }
 
-template<QuantityValue Rep, typename D, typename U>
-[[nodiscard]] constexpr Quantity auto operator/(const Rep& lhs, reference<D, U>)
-{
-  return lhs / quantity<D, U, Rep>::one();
-}
-
 template<Quantity Q1, Quantity Q2>
   requires quantity_value_for_<std::divides<>, typename Q1::rep, typename Q2::rep>
 [[nodiscard]] constexpr Quantity auto operator/(const Q1& lhs, const Q2& rhs)

src/core/include/units/reference.h

@@ -27,6 +27,9 @@
 
 namespace units {
 
+template<Dimension D, UnitOf<D> U, QuantityValue Rep>
+class quantity;
+
 template<Dimension D, UnitOf<D> U>
 struct reference;
 
@@ -92,24 +95,28 @@ struct reference {
   // Hidden Friends
   // Below friend functions are to be found via argument-dependent lookup only
 
-#if !UNITS_COMP_MSVC
+  template<Reference R2>
-  template<QuantityValue Rep, typename D2, typename U2>
+  [[nodiscard]] friend constexpr reference_multiply<reference, R2> operator*(reference, R2) { return {}; }
-  friend constexpr Quantity auto operator*(const Rep& lhs, reference<D2, U2>);
 
-  template<QuantityValue Rep, typename D2, typename U2>
+  template<Reference R2>
-  friend constexpr Quantity auto operator/(const Rep& lhs, reference<D2, U2>);
+  [[nodiscard]] friend constexpr reference_divide<reference, R2> operator/(reference, R2) { return {}; }
-#endif
+
+  template<QuantityValue Rep>
+  [[nodiscard]] friend constexpr Quantity auto operator*(const Rep& lhs, reference)
+  {
+    return quantity<D, U, Rep>(lhs);
+  }
+
+  template<QuantityValue Rep>
+  [[nodiscard]] friend constexpr Quantity auto operator/(const Rep& lhs, reference)
+  {
+    return lhs / quantity<D, U, Rep>::one();
+  }
 
   friend void /*Use `q * (1 * r)` rather than `q * r`.*/ operator*(Quantity auto, reference) = delete;
   friend void /*Use `q / (1 * r)` rather than `q / r`.*/ operator/(Quantity auto, reference) = delete;
 };
 
-template<Reference R1, Reference R2>
-[[nodiscard]] constexpr reference_multiply<R1, R2> operator*(R1, R2) { return {}; }
-
-template<Reference R1, Reference R2>
-[[nodiscard]] constexpr reference_divide<R1, R2> operator/(R1, R2) { return {}; }
-
 // type traits
 namespace detail {