fix #10910, optimize squaring and cubing (#11291)

lib/pure/math.nim

@@ -975,16 +975,21 @@ proc `^`*[T](x: T, y: Natural): T =
   ## * `sqrt proc <#sqrt,float64>`_
   ## * `cbrt proc <#cbrt,float64>`_
   ##
-  ## .. code-block:: nim
+  runnableExamples:
-  ##  echo 2^3  # 8
+    assert -3.0^0 == 1.0
-  ##  echo -2^3 # -8
+    assert -3^1 == -3
-  when compiles(y >= T(0)):
+    assert -3^2 == 9
-    assert y >= T(0)
+    assert -3.0^3 == -27.0
+    assert -3.0^4 == 81.0
+
+  case y
+  of 0: result = 1
+  of 1: result = x
+  of 2: result = x * x
+  of 3: result = x * x * x
   else:
-    assert T(y) >= T(0)
     var (x, y) = (x, y)
     result = 1
-
     while true:
       if (y and 1) != 0:
         result *= x