Stop shadowing ONE (#205)

Our loosening of the overflow safety surface introduced a warning, which
is prominently visible on our [canonical godbolt link]:


![image](https://github.com/aurora-opensource/au/assets/1819744/d55ef99f-7073-4644-9ffc-f75fa3893441)

Oops!  We shadowed the _magnitude_ `ONE` (globally available in the
library) with the _constant_ `ONE`.

The quick fix is to rename the newcomer (which is not publicly visible
anyway) to `UNITY`.

To prevent this problem from recurring, we also add `-Wshadow` to our
regular compiler options.

[canonical godbolt link]: https://godbolt.org/z/KrvfhP4M3

au/math.hh

@@ -238,8 +238,8 @@ constexpr auto int_pow(Quantity<U, R> q) {
 template <typename TargetRep, typename TargetUnits, typename U, typename R>
 constexpr auto inverse_in(TargetUnits target_units, Quantity<U, R> q) {
     using Rep = std::common_type_t<TargetRep, R>;
-    constexpr auto ONE = make_constant(UnitProductT<>{});
-    return static_cast<TargetRep>(ONE.in<Rep>(associated_unit(target_units) * U{}) / q.in(U{}));
+    constexpr auto UNITY = make_constant(UnitProductT<>{});
+    return static_cast<TargetRep>(UNITY.in<Rep>(associated_unit(target_units) * U{}) / q.in(U{}));
 }
 
 //
@@ -270,10 +270,10 @@ constexpr auto inverse_in(TargetUnits target_units, Quantity<U, R> q) {
     // This will fail at compile time for types that can't hold 1'000'000.
     constexpr R threshold = 1'000'000;
 
-    constexpr auto ONE = make_constant(UnitProductT<>{});
+    constexpr auto UNITY = make_constant(UnitProductT<>{});
 
     static_assert(
-        ONE.in<R>(associated_unit(target_units) * U{}) >= threshold ||
+        UNITY.in<R>(associated_unit(target_units) * U{}) >= threshold ||
             std::is_floating_point<R>::value,
         "Dangerous inversion risking truncation to 0; must supply explicit Rep if truly desired");
 

build/copts.bzl

@@ -30,6 +30,7 @@ BASE_CLANG_COPTS = [
     # Diagnostics
     "-fcolor-diagnostics",
     "-Wall",
+    "-Wshadow",
     "-Wthread-safety",
     "-Wself-assign",
 ]