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/samples/test/samples/collections/iterables.kt

@@ -26,7 +26,7 @@ class Iterables {
         @Sample
         fun iterable() {
             val iterable = Iterable {
-                buildIterator {
+                iterator {
                     yield(42)
                     yieldAll(1..5 step 2)
                 }

libraries/stdlib/samples/test/samples/collections/sequences.kt

@@ -111,7 +111,7 @@ class Sequences {
 
         @Sample
         fun buildFibonacciSequence() {
-            fun fibonacci() = buildSequence {
+            fun fibonacci() = sequence {
                 var terms = Pair(0, 1)
 
                 // this sequence is infinite
@@ -126,7 +126,7 @@ class Sequences {
 
         @Sample
         fun buildSequenceYieldAll() {
-            val sequence = buildSequence {
+            val sequence = sequence {
                 val start = 0
                 // yielding a single value
                 yield(start)
@@ -145,7 +145,7 @@ class Sequences {
             val wrappedCollection = object : AbstractCollection<Any>() {
                 override val size: Int = collection.size + 2
 
-                override fun iterator(): Iterator<Any> = buildIterator {
+                override fun iterator(): Iterator<Any> = iterator {
                     yield("first")
                     yieldAll(collection)
                     yield("last")