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/common/hpack-dec.h b/include/common/hpack-dec.h
index b03398a..71039d3 100644
--- a/include/common/hpack-dec.h
+++ b/include/common/hpack-dec.h
@@ -34,6 +34,7 @@
 #include <common/hpack-tbl.h>
 
 int hpack_decode_frame(struct hpack_dht *dht, const uint8_t *raw, uint32_t len,
-                       struct http_hdr *list, int list_size, struct chunk *tmp);
+                       struct http_hdr *list, int list_size,
+                       struct buffer *tmp);
 
 #endif /* _COMMON_HPACK_DEC_H */