Migrate units from ratio to Magnitude

This commit is huge, but hopefully the cognitive load is not too bad.
The bulk of this commit is just some fairly mechanical updates from
`ratio` to `Magnitude`.  Other things to call out:

- `UnitRatio` goes away.  We don't need this concept, because Magnitude
  can't even _represent_ anything that doesn't satisfy it.

- I commented out some formatting test cases where the precise
  expression changes, but the number is completely equivalent.  We will
  need to decide how we want to handle Magnitude formatting as a
  separate, follow-on task.  But at least Magnitude gives us all the
  tools we'll need to do so!

src/core/include/units/unit.h

@@ -28,6 +28,7 @@
 // IWYU pragma: begin_exports
 #include <units/bits/derived_unit.h>
 #include <units/bits/external/fixed_string.h>
+#include <units/magnitude.h>
 #include <units/prefix.h>
 #include <units/ratio.h>
 #include <units/symbol_text.h>
@@ -47,20 +48,18 @@ namespace units {
  * (i.e. all length units are expressed in terms of meter, all mass units are expressed in
  * terms of gram, ...)
  *
- * @tparam R a ratio of a reference unit
+ * @tparam M a Magnitude representing the (relative) size of this unit
  * @tparam U a unit to use as a reference for this dimension
  */
-template<ratio R, typename U>
-  requires UnitRatio<R>
-struct scaled_unit : downcast_base<scaled_unit<R, U>> {
-  static constexpr ::units::ratio ratio = R;
+template<Magnitude auto M, typename U>
+struct scaled_unit : downcast_base<scaled_unit<M, U>> {
+  static constexpr ::units::ratio ratio = as_ratio(M);
+  static constexpr Magnitude auto mag = M;
   using reference = U;
 };
 
-template<Dimension D, auto R>
-// template<Dimension D, ratio R>  // TODO: GCC crash!!!
-  requires UnitRatio<R>
-using downcast_unit = downcast<scaled_unit<R, typename dimension_unit<D>::reference>>;
+template<Dimension D, Magnitude auto M>
+using downcast_unit = downcast<scaled_unit<M, typename dimension_unit<D>::reference>>;
 
 template<Unit U1, Unit U2>
 struct same_unit_reference : is_same<typename U1::reference, typename U2::reference> {};
@@ -74,7 +73,7 @@ struct same_unit_reference : is_same<typename U1::reference, typename U2::refere
  * @tparam Child inherited class type used by the downcasting facility (CRTP Idiom)
  */
 template<typename Child>
-struct unit : downcast_dispatch<Child, scaled_unit<ratio(1), Child>> {
+struct unit : downcast_dispatch<Child, scaled_unit<as_magnitude<1>(), Child>> {
   static constexpr bool is_named = false;
   using prefix_family = no_prefix;
 };
@@ -92,7 +91,7 @@ struct unit : downcast_dispatch<Child, scaled_unit<ratio(1), Child>> {
  * @tparam PF no_prefix or a type of prefix family
  */
 template<typename Child, basic_symbol_text Symbol, PrefixFamily PF>
-struct named_unit : downcast_dispatch<Child, scaled_unit<ratio(1), Child>> {
+struct named_unit : downcast_dispatch<Child, scaled_unit<as_magnitude<1>(), Child>> {
   static constexpr bool is_named = true;
   static constexpr auto symbol = Symbol;
   using prefix_family = PF;
@@ -109,12 +108,11 @@ struct named_unit : downcast_dispatch<Child, scaled_unit<ratio(1), Child>> {
  * @tparam Child inherited class type used by the downcasting facility (CRTP Idiom)
  * @tparam Symbol a short text representation of the unit
  * @tparam PF no_prefix or a type of prefix family
- * @tparam R a scale to apply to U
+ * @tparam M the Magnitude by which to scale U
  * @tparam U a reference unit to scale
  */
-template<typename Child, basic_symbol_text Symbol, PrefixFamily PF, ratio R, Unit U>
-  requires UnitRatio<R>
-struct named_scaled_unit : downcast_dispatch<Child, scaled_unit<R * U::ratio, typename U::reference>> {
+template<typename Child, basic_symbol_text Symbol, PrefixFamily PF, Magnitude auto M, Unit U>
+struct named_scaled_unit : downcast_dispatch<Child, scaled_unit<M * U::mag, typename U::reference>> {
   static constexpr bool is_named = true;
   static constexpr auto symbol = Symbol;
   using prefix_family = PF;
@@ -133,7 +131,7 @@ struct named_scaled_unit : downcast_dispatch<Child, scaled_unit<R * U::ratio, ty
  */
 template<typename Child, Prefix P, Unit U>
   requires U::is_named && std::same_as<typename P::prefix_family, typename U::prefix_family>
-struct prefixed_unit : downcast_dispatch<Child, scaled_unit<P::ratio * U::ratio, typename U::reference>> {
+struct prefixed_unit : downcast_dispatch<Child, scaled_unit<P::mag * U::mag, typename U::reference>> {
   static constexpr bool is_named = true;
   static constexpr auto symbol = P::symbol + U::symbol;
   using prefix_family = no_prefix;