factor: Introduce a type alias for exponents

This way, we can easily replace u8 with a larger type when moving to support
larger integers.

src/uu/factor/src/factor.rs

@@ -61,6 +61,9 @@ impl PartialEq for Decomposition {
 
         true
     }
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct Factors {
+    f: BTreeMap<u64, Exponent>,
 }
 impl Eq for Decomposition {}