maths: add integer consteval logarithm

maths.hpp

@@ -214,6 +214,23 @@ namespace cruft {
     }
 
 
+    /// Naively calculates the integer log of `val` in `base`, rounded down.
+    ///
+    /// We deliberately restrict this to consteval to limit unexpected issues
+    /// with runtime performance given the simplistic construction.
+    ///
+    /// It's useful for sizing temporary arrays.
+    template <concepts::integral T>
+    consteval T
+    ilog (T val, T base)
+    {
+        T tally = 0;
+        while (val /= base)
+            ++tally;
+        return tally;
+    }
+
+
     ///////////////////////////////////////////////////////////////////////////////
     /// round T up to the nearest multiple of U
     template <concepts::integral T, concepts::integral U>