KT-15363 Collection Builders

Added container builders for lists, sets, and maps.
The new experimental type inference only works for the simple builders
with a single generic type. The versions with nullability and the maps
still require explicit specification of the types. Obviously explicit
specification is required for all users who are not using the new
experimental inference. Improving the type inference is something that
should be done separately and many things – including these builders –
will benefit from it, however, this is not a blocker for these builders
in my opinion.

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

@@ -0,0 +1,51 @@
+package samples.collections
+
+import samples.*
+
+@RunWith(Enclosed::class)
+class Builders {
+    class Lists {
+        @Sample
+        fun buildListSample() {
+            val x = listOf('b', 'c')
+
+            val y = buildList(x.size + 2) {
+                add('a')
+                addAll(x)
+                add('d')
+            }
+
+            assertPrints(y, "[a, b, c, d]")
+        }
+    }
+
+    class Sets {
+        @Sample
+        fun buildSetSample() {
+            val x = setOf('b', 'c')
+
+            val y = buildSet(x.size + 2) {
+                add('a')
+                addAll(x)
+                add('d')
+            }
+
+            assertPrints(y, "[a, b, c, d]")
+        }
+    }
+
+    class Maps {
+        @Sample
+        fun buildMapSample() {
+            val x = mapOf('b' to 2, 'c' to 3)
+
+            val y = buildMap<Char, Int>(x.size + 2) {
+                put('a', 1)
+                putAll(x)
+                put('d', 4)
+            }
+
+            assertPrints(y, "{a=1, b=2, c=3, d=4}")
+        }
+    }
+}