rationals: support Rational[SomeUnsignedInt] (#23046)

fixes #22227
rationale:
    - `3u - 4u` is supported why not`3u.toRational - 4u.toRational`
- all of rationals' api is on SomeInteger, looks like unsigned is
declared as supported
  - math on unsigned rationals is meaningful and useful.

lib/pure/rationals.nim

@@ -40,16 +40,16 @@ func reduce*[T: SomeInteger](x: var Rational[T]) =
     reduce(r)
     doAssert r.num == 1
     doAssert r.den == 2
-
+  if x.den == 0:
+    raise newException(DivByZeroDefect, "division by zero")
   let common = gcd(x.num, x.den)
   if x.den > 0:
     x.num = x.num div common
     x.den = x.den div common
-  elif x.den < 0:
-    x.num = -x.num div common
-    x.den = -x.den div common
-  else:
-    raise newException(DivByZeroDefect, "division by zero")
+  when T isnot SomeUnsignedInt:
+    if x.den < 0:
+      x.num = -x.num div common
+      x.den = -x.den div common
 
 func initRational*[T: SomeInteger](num, den: T): Rational[T] =
   ## Creates a new rational number with numerator `num` and denominator `den`.

tests/stdlib/trationals.nim

@@ -10,6 +10,7 @@ template main() =
     z = Rational[int](num: 0, den: 1)
     o = initRational(num = 1, den = 1)
     a = initRational(1, 2)
+    u = 3u // 2
     b = -1 // -2
     m1 = -1 // 1
     tt = 10 // 2
@@ -104,5 +105,13 @@ template main() =
   when sizeof(int) == 8:
     doAssert almostEqual(PI.toRational.toFloat, PI)
 
+  # unsigned
+  doAssert u == u
+  doAssert u + u == 3u // 1
+  doAssert 3u.toRational - u == u
+  doAssert u * 2 == 3u // 1
+
+
+
 static: main()
 main()