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/tools/kotlin-stdlib-gen/src/templates/Generators.kt

@@ -944,7 +944,7 @@ object Generators : TemplateGroupBase() {
         }
         body(Sequences) {
             """
-            return buildSequence result@ {
+            return sequence result@ {
                 val iterator = iterator()
                 if (!iterator.hasNext()) return@result
                 var current = iterator.next()