Rename sequence and iterator builder functions and their scope class

This introduces new functions instead of the existing sequence builders:
- `sequence` instead of `buildSequence`
- `iterator` instead of `buildIterator`
- `SequenceScope` instead of `SequenceBuilder`

The old functions were deprecated with error and made inline-only, and `SequenceBuilder` has been
made a deprecated typealias to `SequenceScope`.

Move sequence builders to `SequencesKt` facade class.

Replace sequence builder usages in stdlib and samples.

#KT-26678

libraries/stdlib/src/kotlin/collections/Sequences.kt

@@ -63,7 +63,7 @@ public inline fun <T> Sequence<T>?.orEmpty(): Sequence<T> = this ?: emptySequenc
  * @sample samples.collections.Sequences.Usage.sequenceIfEmpty
  */
 @SinceKotlin("1.3")
-public fun <T> Sequence<T>.ifEmpty(defaultValue: () -> Sequence<T>): Sequence<T> = buildSequence {
+public fun <T> Sequence<T>.ifEmpty(defaultValue: () -> Sequence<T>): Sequence<T> = sequence {
     val iterator = this@ifEmpty.iterator()
     if (iterator.hasNext()) {
         yieldAll(iterator)
@@ -588,7 +588,7 @@ public fun <T> Sequence<T>.constrainOnce(): Sequence<T> {
  * The returned sequence is constrained to be iterated only once.
  *
  * @see constrainOnce
- * @see kotlin.coroutines.experimental.buildSequence
+ * @see kotlin.sequences.sequence
  *
  * @sample samples.collections.Sequences.Building.generateSequence
  */
@@ -605,7 +605,7 @@ public fun <T : Any> generateSequence(nextFunction: () -> T?): Sequence<T> {
  *
  * The sequence can be iterated multiple times, each time starting with [seed].
  *
- * @see kotlin.coroutines.experimental.buildSequence
+ * @see kotlin.sequences.sequence
  *
  * @sample samples.collections.Sequences.Building.generateSequenceWithSeed
  */
@@ -625,7 +625,7 @@ public fun <T : Any> generateSequence(seed: T?, nextFunction: (T) -> T?): Sequen
  *
  * The sequence can be iterated multiple times.
  *
- * @see kotlin.coroutines.experimental.buildSequence
+ * @see kotlin.sequences.sequence
  *
  * @sample samples.collections.Sequences.Building.generateSequenceWithLazySeed
  */

libraries/stdlib/src/kotlin/collections/SlidingWindow.kt

@@ -23,7 +23,7 @@ internal fun <T> Sequence<T>.windowedSequence(size: Int, step: Int, partialWindo
 
 internal fun <T> windowedIterator(iterator: Iterator<T>, size: Int, step: Int, partialWindows: Boolean, reuseBuffer: Boolean): Iterator<List<T>> {
     if (!iterator.hasNext()) return EmptyIterator
-    return buildIterator<List<T>> {
+    return iterator<List<T>> {
         val gap = step - size
         if (gap >= 0) {
             var buffer = ArrayList<T>(size)