[SPARK-19089][SQL] Add support for nested sequences

## What changes were proposed in this pull request?

Replaced specific sequence encoders with generic sequence encoder to enable nesting of sequences.

Does not add support for nested arrays as that cannot be solved in this way.

## How was this patch tested?

```bash
build/mvn -DskipTests clean package && dev/run-tests
```

Additionally in Spark shell:

```
scala> Seq(Seq(Seq(1))).toDS.collect()
res0: Array[Seq[Seq[Int]]] = Array(List(List(1)))
```

Author: Michal Senkyr <mike.senkyr@gmail.com>

Closes apache#18011 from michalsenkyr/dataset-seq-nested.

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

@@ -111,93 +111,60 @@ abstract class SQLImplicits extends LowPrioritySQLImplicits {
 
   /**
    * @since 1.6.1
-   * @deprecated use [[newIntSequenceEncoder]]
+   * @deprecated use [[newSequenceEncoder]]
    */
   def newIntSeqEncoder: Encoder[Seq[Int]] = ExpressionEncoder()
 
   /**
    * @since 1.6.1
-   * @deprecated use [[newLongSequenceEncoder]]
+   * @deprecated use [[newSequenceEncoder]]
    */
   def newLongSeqEncoder: Encoder[Seq[Long]] = ExpressionEncoder()
 
   /**
    * @since 1.6.1
-   * @deprecated use [[newDoubleSequenceEncoder]]
+   * @deprecated use [[newSequenceEncoder]]
    */
   def newDoubleSeqEncoder: Encoder[Seq[Double]] = ExpressionEncoder()
 
   /**
    * @since 1.6.1
-   * @deprecated use [[newFloatSequenceEncoder]]
+   * @deprecated use [[newSequenceEncoder]]
    */
   def newFloatSeqEncoder: Encoder[Seq[Float]] = ExpressionEncoder()
 
   /**
    * @since 1.6.1
-   * @deprecated use [[newByteSequenceEncoder]]
+   * @deprecated use [[newSequenceEncoder]]
    */
   def newByteSeqEncoder: Encoder[Seq[Byte]] = ExpressionEncoder()
 
   /**
    * @since 1.6.1
-   * @deprecated use [[newShortSequenceEncoder]]
+   * @deprecated use [[newSequenceEncoder]]
    */
   def newShortSeqEncoder: Encoder[Seq[Short]] = ExpressionEncoder()
 
   /**
    * @since 1.6.1
-   * @deprecated use [[newBooleanSequenceEncoder]]
+   * @deprecated use [[newSequenceEncoder]]
    */
   def newBooleanSeqEncoder: Encoder[Seq[Boolean]] = ExpressionEncoder()
 
   /**
    * @since 1.6.1
-   * @deprecated use [[newStringSequenceEncoder]]
+   * @deprecated use [[newSequenceEncoder]]
    */
   def newStringSeqEncoder: Encoder[Seq[String]] = ExpressionEncoder()
 
   /**
    * @since 1.6.1
-   * @deprecated use [[newProductSequenceEncoder]]
+   * @deprecated use [[newSequenceEncoder]]
    */
-  implicit def newProductSeqEncoder[A <: Product : TypeTag]: Encoder[Seq[A]] = ExpressionEncoder()
+  def newProductSeqEncoder[A <: Product : TypeTag]: Encoder[Seq[A]] = ExpressionEncoder()
 
   /** @since 2.2.0 */
-  implicit def newIntSequenceEncoder[T <: Seq[Int] : TypeTag]: Encoder[T] =
-    ExpressionEncoder()
-
-  /** @since 2.2.0 */
-  implicit def newLongSequenceEncoder[T <: Seq[Long] : TypeTag]: Encoder[T] =
-    ExpressionEncoder()
-
-  /** @since 2.2.0 */
-  implicit def newDoubleSequenceEncoder[T <: Seq[Double] : TypeTag]: Encoder[T] =
-    ExpressionEncoder()
-
-  /** @since 2.2.0 */
-  implicit def newFloatSequenceEncoder[T <: Seq[Float] : TypeTag]: Encoder[T] =
-    ExpressionEncoder()
-
-  /** @since 2.2.0 */
-  implicit def newByteSequenceEncoder[T <: Seq[Byte] : TypeTag]: Encoder[T] =
-    ExpressionEncoder()
-
-  /** @since 2.2.0 */
-  implicit def newShortSequenceEncoder[T <: Seq[Short] : TypeTag]: Encoder[T] =
-    ExpressionEncoder()
-
-  /** @since 2.2.0 */
-  implicit def newBooleanSequenceEncoder[T <: Seq[Boolean] : TypeTag]: Encoder[T] =
-    ExpressionEncoder()
-
-  /** @since 2.2.0 */
-  implicit def newStringSequenceEncoder[T <: Seq[String] : TypeTag]: Encoder[T] =
-    ExpressionEncoder()
-
-  /** @since 2.2.0 */
-  implicit def newProductSequenceEncoder[T <: Seq[Product] : TypeTag]: Encoder[T] =
-    ExpressionEncoder()
+  implicit def newSequenceEncoder[T <: Seq[_] : TypeTag]: Encoder[T] = ExpressionEncoder()
 
   // Arrays
 

sql/core/src/test/scala/org/apache/spark/sql/DatasetPrimitiveSuite.scala

@@ -258,6 +258,11 @@ class DatasetPrimitiveSuite extends QueryTest with SharedSQLContext {
       ListClass(List(1)) -> Queue("test" -> SeqClass(Seq(2))))
   }
 
+  test("nested sequences") {
+    checkDataset(Seq(Seq(Seq(1))).toDS(), Seq(Seq(1)))
+    checkDataset(Seq(List(Queue(1))).toDS(), List(Queue(1)))
+  }
+
   test("package objects") {
     import packageobject._
     checkDataset(Seq(PackageClass(1)).toDS(), PackageClass(1))