proposal: math/big: Rat.SetUint64

Please consider adding `Rat.SetUint64` in addition to [Rat.SetInt64](https://golang.org/pkg/math/big/#Rat.SetInt64) as there is currently no way of setting by `uint64` integers. This is similar to [Int.SetInt64](https://golang.org/pkg/math/big/#Int.SetInt64) and [Int.SetUint64](https://golang.org/pkg/math/big/#Int.SetUint64).