feat: improve types readability by eliminating extraneous () for a value of a type for prefixes

[mpusz]

This PR improves types readability.

-error: could not convert 'speed' from 'quantity<derived_unit<si::kilo_<si::metre()>, per<non_si::hour>>()>'
+error: could not convert 'speed' from 'quantity<derived_unit<si::kilo_<si::metre>, per<non_si::hour>>()>'
       to 'quantity<si::metre()>'
   50 |   std::println("TTG: {:{%N:.6} %U}",
      |                time_to_goal(speed, distance_m));
      |                                      ^~~~~
      |                                      |
-     |                                      quantity<derived_unit<si::kilo_<si::metre()>, per<non_si::hour>>()>
+     |                                      quantity<derived_unit<si::kilo_<si::metre>, per<non_si::hour>>()>
Compiler returned: 1

error: no matching function for call to 'time_to_goal'
   22 | const quantity ttg = time_to_goal(half_marathon_distance, pace);
      |                      ^~~~~~~~~~~~
-note: candidate template ignored: constraints not satisfied [with distance:auto = quantity<kilo_<metre{{}}>{}, double>,
-                                                                  speed:auto = quantity<derived_unit<second, per<kilo_<metre{{}}>>>{}, double>]
+note: candidate template ignored: constraints not satisfied [with distance:auto = quantity<kilo_<metre>{}, double>,
+                                                                  speed:auto = quantity<derived_unit<second, per<kilo_<metre>>>{}, double>]
   12 | QuantityOf<isq::time> auto time_to_goal(QuantityOf<isq::length> auto distance,
      |                            ^
-note: because 'QuantityOf<quantity<derived_unit<si::second, per<si::kilo_<si::metre{{}}>>>{{{}}}>, isq::speed>' evaluated to false
+note: because 'QuantityOf<quantity<derived_unit<si::second, per<si::kilo_<si::metre>>>{{{}}}>, isq::speed>' evaluated to false
   13 |                                         QuantityOf<isq::speed> auto speed)
      |                                         ^
-note: because 'QuantitySpecOf<std::remove_const_t<decltype(get_quantity_spec(derived_unit<second, per<kilo_<metre{{}}>>>{}))>, struct speed{{{}}}>' evaluated to false
+note: because 'QuantitySpecOf<std::remove_const_t<decltype(get_quantity_spec(derived_unit<second, per<kilo_<metre>>>{}))>, struct speed{{{}}}>' evaluated to false
   77 |                    QuantitySpecOf<std::remove_const_t<decltype(get_quantity_spec(T{}))>, V>);
      |                    ^
note: because 'implicitly_convertible(kind_of_<derived_quantity_spec<isq::time, per<isq::length>>{{}, {{}}}>{}, struct speed{{{}}})' evaluated to false
  147 |   QuantitySpec<T> && QuantitySpec<std::remove_const_t<decltype(QS)>> && implicitly_convertible(T{}, QS) &&
      |                                                                         ^
1 error generated.
Compiler returned: 1

Is it worth it?

[chiphogg]

A nice bit of polish, tidying up error messages without complicating things for end users. Thanks!