Be a little bit more careful about what items we do and don't include, start adding modular math into the system.

src/unsigned/modmul.rs

@@ -0,0 +1,16 @@
+pub trait ModMul<T> {
+    fn modmul(&self, x: &Self, m: &T) -> Self;
+}
+
+macro_rules! modmul_impls {
+    ($name: ident, $dbl: ident) => {
+        impl ModMul<$name> for $name {
+            fn modmul(&self, x: &$name, m: &$name) -> $name {
+                let mulres      = (self as &$name) * x;
+                let bigm        = $dbl::from(m);
+                let (_, bigres) = mulres.divmod(&bigm);
+                $name::from(bigres)
+            }
+        }
+    };
+}