Optimize checked_ilog when base is a power of two

Make use of the identity `log(base, n) == log(2, n) / log(2, base)`
to optimize when `base` is a power of two.

Author: Kmeakin

library/core/src/num/uint_macros.rs

@@ -1573,7 +1573,15 @@ macro_rules! uint_impl {
             if core::intrinsics::is_val_statically_known(base) {
                 if base == 2 {
                     return self.checked_ilog2();
-                } else if base == 10 {
+                }
+                // change of base: log(base, n) == log(2, n) / log(2, base)
+                if base.is_power_of_two() {
+                    return match self.checked_ilog2() {
+                        Some(log2) => Some(log2 / base.ilog2()),
+                        None => None,
+                    };
+                }
+                if base == 10 {
                     return self.checked_ilog10();
                 }
             }