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/src/hpack-enc.c b/src/hpack-enc.c
index c6bfce7..70d2b12 100644
--- a/src/hpack-enc.c
+++ b/src/hpack-enc.c
@@ -76,7 +76,8 @@
 /* Tries to encode header whose name is <n> and value <v> into the chunk <out>.
  * Returns non-zero on success, 0 on failure (buffer full).
  */
-int hpack_encode_header(struct chunk *out, const struct ist n, const struct ist v)
+int hpack_encode_header(struct buffer *out, const struct ist n,
+			const struct ist v)
 {
 	int len = out->data;
 	int size = out->size;