util/util-core: Increase visibility of Buf.Composite

When Bufs are built by concatenation they are usually represented as a `Buf.Composite`.
Since this class is private, it is difficult to consume the underlying discontiguous Bufs without
copying. Making the class public allows constructions like:
```
  def consume(buf: Buf)(eat: Array[Byte] => Unit): Unit = buf match {
    case array: Buf.ByteArray => eat(array.unsafeRawByteArray) // [unsafeArrayOffset + length]
    case Buf.ByteBuffer(bb) if bb.hasArray => eat(bb.array()) // [+ bb.arrayOffset()]
    case Buf.Composite(bufs, _) => bufs.foreach(consume(_)(eat))
    case _ => eat(Buf.ByteArray.Owned.extract(buf))
  }
```

Differential Revision: https://phabricator.twitter.biz/D1198118

Author: keremnc

util-core/src/main/scala/com/twitter/io/Buf.scala

@@ -404,7 +404,7 @@ object Buf {
     def length: Int = 3
   }
 
-  private final case class Composite(bufs: IndexedSeq[Buf], length: Int) extends Buf {
+  final case class Composite(bufs: IndexedSeq[Buf], length: Int) extends Buf {
     // the factory method requires non-empty Bufs
     override def isEmpty: Boolean = false