maths: use mult rather than negation in constexpr

util::digits was triggering signed overflow warnings under gcc. By using
negative multiplication rather than explicit negation we get the same
behaviour, sans warning.

maths.hpp

@@ -312,7 +312,7 @@ namespace util {
     digits (ValueT value, BaseT base) noexcept
     {
         if (value < 0)
-            return digits (-value, base);
+            value *= -1;
 
         unsigned tally = 1;
         while (value /= base)