Replace reduce with bespoke implementation

If _this_ isn't `constexpr` compatible, I'm going to propose removing
support for the MacOS clang build.

src/core/include/units/bits/prime.h

@@ -101,7 +101,11 @@ constexpr auto coprimes_up_to(std::size_t n, const std::array<std::size_t, N>& b
 template<std::size_t N>
 constexpr std::size_t product(const std::array<std::size_t, N>& values)
 {
-  return std::reduce(values.begin(), values.end(), std::size_t{1}, std::multiplies{});
+  std::size_t product = 1;
+  for (const auto& v : values) {
+    product *= v;
+  }
+  return product;
 }
 
 // A configurable instantiation of the "wheel factorization" algorithm [1] for prime numbers.