math/big: implement fast path in Float.SetRat if argument is integer

Change-Id: Ib82500e198b86e9fade278c7eea7a4b0c6b0b2e1
Reviewed-on: https://go-review.googlesource.com/4921Reviewed-by: Rob Pike <r@golang.org>

src/math/big/float.go

@@ -582,7 +582,9 @@ func (z *Float) SetInt(x *Int) *Float {
 // If z's precision is 0, it is changed to the largest of a.BitLen(),
 // b.BitLen(), or 64; with x = a/b.
 func (z *Float) SetRat(x *Rat) *Float {
-	// TODO(gri) can be more efficient if x is an integer
+	if x.IsInt() {
+		return z.SetInt(x.Num())
+	}
 	var a, b Float
 	a.SetInt(x.Num())
 	b.SetInt(x.Denom())