[SPARK-11876][SQL] Support printSchema in DataSet API

DataSet APIs look great! However, I am lost when doing multiple level joins.  For example,
```
val ds1 = Seq(("a", 1), ("b", 2)).toDS().as("a")
val ds2 = Seq(("a", 1), ("b", 2)).toDS().as("b")
val ds3 = Seq(("a", 1), ("b", 2)).toDS().as("c")

ds1.joinWith(ds2, $"a._2" === $"b._2").as("ab").joinWith(ds3, $"ab._1._2" === $"c._2").printSchema()
```

The printed schema is like
```
root
 |-- _1: struct (nullable = true)
 |    |-- _1: struct (nullable = true)
 |    |    |-- _1: string (nullable = true)
 |    |    |-- _2: integer (nullable = true)
 |    |-- _2: struct (nullable = true)
 |    |    |-- _1: string (nullable = true)
 |    |    |-- _2: integer (nullable = true)
 |-- _2: struct (nullable = true)
 |    |-- _1: string (nullable = true)
 |    |-- _2: integer (nullable = true)
```

Personally, I think we need the printSchema function. Sometimes, I do not know how to specify the column, especially when their data types are mixed. For example, if I want to write the following select for the above multi-level join, I have to know the schema:
```
newDS.select(expr("_1._2._2 + 1").as[Int]).collect()
```

marmbrus rxin cloud-fan  Do you have the same feeling?

Author: gatorsmile <gatorsmile@gmail.com>

Closes #9855 from gatorsmile/printSchemaDataSet.

sql/core/src/main/scala/org/apache/spark/sql/DataFrame.scala

@@ -299,15 +299,6 @@ class DataFrame private[sql](
    */
   def columns: Array[String] = schema.fields.map(_.name)
 
-  /**
-   * Prints the schema to the console in a nice tree format.
-   * @group basic
-   * @since 1.3.0
-   */
-  // scalastyle:off println
-  def printSchema(): Unit = println(schema.treeString)
-  // scalastyle:on println
-
   /**
    * Returns true if the `collect` and `take` methods can be run locally
    * (without any Spark executors).

sql/core/src/main/scala/org/apache/spark/sql/execution/Queryable.scala

@@ -37,6 +37,15 @@ private[sql] trait Queryable {
     }
   }
 
+  /**
+   * Prints the schema to the console in a nice tree format.
+   * @group basic
+   * @since 1.3.0
+   */
+  // scalastyle:off println
+  def printSchema(): Unit = println(schema.treeString)
+  // scalastyle:on println
+
   /**
    * Prints the plans (logical and physical) to the console for debugging purposes.
    * @since 1.3.0