MAJOR: chunks: replace struct chunk with struct buffer

Now all the code used to manipulate chunks uses a struct buffer instead.
The functions are still called "chunk*", and some of them will progressively
move to the generic buffer handling code as they are cleaned up.
diff --git a/include/proto/stats.h b/include/proto/stats.h
index ac893b8..02a3506 100644
--- a/include/proto/stats.h
+++ b/include/proto/stats.h
@@ -101,9 +101,10 @@
 extern struct applet http_stats_applet;
 
 void stats_io_handler(struct stream_interface *si);
-int stats_emit_raw_data_field(struct chunk *out, const struct field *f);
-int stats_emit_typed_data_field(struct chunk *out, const struct field *f);
-int stats_emit_field_tags(struct chunk *out, const struct field *f, char delim);
+int stats_emit_raw_data_field(struct buffer *out, const struct field *f);
+int stats_emit_typed_data_field(struct buffer *out, const struct field *f);
+int stats_emit_field_tags(struct buffer *out, const struct field *f,
+			  char delim);
 
 #endif /* _PROTO_STATS_H */