Feat: Some more key/value dataset functions

[Jolanrensen]

I don't have time to implement and write tests currently, but I thought these would be useful to have as well:

@JvmName("sortByKeyTuple2")
fun <T1, T2> Dataset<Tuple2<T1, T2>>.sortByKey(): Dataset<Tuple2<T1, T2>> = sort("_1")

@JvmName("sortByValueTuple2")
fun <T1, T2> Dataset<Tuple2<T1, T2>>.sortByValue(): Dataset<Tuple2<T1, T2>> = sort("_2")

@JvmName("sortByKeyArity2")
fun <T1, T2> Dataset<Arity2<T1, T2>>.sortByKey(): Dataset<Arity2<T1, T2>> = sort("_1")

@JvmName("sortByValueArity2")
fun <T1, T2> Dataset<Arity2<T1, T2>>.sortByValue(): Dataset<Arity2<T1, T2>> = sort("_2")

fun <T1, T2> Dataset<Pair<T1, T2>>.sortByKey(): Dataset<Pair<T1, T2>> = sort("first")

fun <T1, T2> Dataset<Pair<T1, T2>>.sortByValue(): Dataset<Pair<T1, T2>> = sort("second")

Feel free to make a pull request for this if you want it too.