fixed_string: use pointers for copy to storage

fixed_string.hpp

@@ -26,7 +26,7 @@ namespace cruft {
         constexpr
         fixed_string (char const _value[N+1]) noexcept
         {
-            std::copy_n (std::begin (_value), N, std::begin (value));
+            std::copy_n (_value, N, value);
         }
 
         constexpr operator char const* () const { return value; }