Introduce unsafe API for bulk read/write ops.

The API aimed to facilitate integration
with other frameworks and libraries.

Implemented API was initially described in the "Bulk API" subsection of
#135 (comment)

Author: fzhinkin

core/api/kotlinx-io-core.api

@@ -1,4 +1,12 @@
 public final class kotlinx/io/Buffer : kotlinx/io/Sink, kotlinx/io/Source {
+	public final fun #int#getHead ()Lkotlinx/io/Segment;
+	public final fun #int#getSize ()J
+	public final fun #int#getTail ()Lkotlinx/io/Segment;
+	public final fun #int#recycleTail ()V
+	public final fun #int#setHead (Lkotlinx/io/Segment;)V
+	public final fun #int#setSize (J)V
+	public final fun #int#setTail (Lkotlinx/io/Segment;)V
+	public final fun #int#writableSegment (I)Lkotlinx/io/Segment;
 	public fun <init> ()V
 	public final fun clear ()V
 	public fun close ()V