unbreak on older Rust

Author: strake

src/power.rs

@@ -115,4 +115,6 @@ macro_rules! impl_Power {
     ($($t:ty),*) => { $(impl_Power!($t);)* };
 }
 
-impl_Power!(usize, u8, u16, u32, u64, u128);
+impl_Power!(usize, u8, u16, u32, u64);
+#[cfg(has_i128)]
+impl_Power!(u128);