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/common/src/generated/_Sequences.kt

@@ -1658,7 +1658,7 @@ public fun <T> Sequence<T>.zipWithNext(): Sequence<Pair<T, T>> {
  */
 @SinceKotlin("1.2")
 public fun <T, R> Sequence<T>.zipWithNext(transform: (a: T, b: T) -> R): Sequence<R> {
-    return buildSequence result@ {
+    return sequence result@ {
         val iterator = iterator()
         if (!iterator.hasNext()) return@result
         var current = iterator.next()