[SPARK-6326][SQL] Improve castStruct to be faster

Current `castStruct` should be very slow. This pr slightly improves it.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes apache#5017 from viirya/faster_caststruct and squashes the following commits:

385d5b0 [Liang-Chi Hsieh] Further improved.
746fcfb [Liang-Chi Hsieh] Make castStruct faster.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala

@@ -394,10 +394,17 @@ case class Cast(child: Expression, dataType: DataType) extends UnaryExpression w
     val casts = from.fields.zip(to.fields).map {
       case (fromField, toField) => cast(fromField.dataType, toField.dataType)
     }
-    // TODO: This is very slow!
-    buildCast[Row](_, row => Row(row.toSeq.zip(casts).map {
-      case (v, cast) => if (v == null) null else cast(v)
-    }: _*))
+    // TODO: Could be faster?
+    val newRow = new GenericMutableRow(from.fields.size)
+    buildCast[Row](_, row => {
+      var i = 0
+      while (i < row.length) {
+        val v = row(i)
+        newRow.update(i, if (v == null) null else casts(i)(v))
+        i += 1
+      }
+      newRow.copy()
+    })
   }
 
   private[this] def cast(from: DataType, to: DataType): Any => Any = to match {