CLEANUP: Use istadv(const struct ist, const size_t) whenever possible

Refactoring performed with the following Coccinelle patch:

    @@
    struct ist i;
    expression e;
    @@

    - i.ptr += e;
    - i.len -= e;
    + i = istadv(i, e);

src/cache.c

@@ -620,8 +620,7 @@ cache_store_http_payload(struct stream *s, struct filter *filter, struct http_ms
 
 			case HTX_BLK_DATA:
 				v = htx_get_blk_value(htx, blk);
-				v.ptr += offset;
-				v.len -= offset;
+				v = istadv(v, offset);
 				if (v.len > len)
 					v.len = len;
 

src/flt_trace.c

@@ -143,8 +143,7 @@ trace_htx_hexdump(struct htx *htx, unsigned int offset, unsigned int len)
 		}
 
 		v = htx_get_blk_value(htx, blk);
-		v.ptr += offset;
-		v.len -= offset;
+		v = istadv(v, offset);
 		offset = 0;
 
 		if (v.len > len)

src/htx.c

@@ -543,8 +543,7 @@ struct htx_blk *htx_add_data_atonce(struct htx *htx, struct ist data)
 		blk = tailblk;
 		goto end;
 	}
-	data.ptr += len;
-	data.len -= len;
+	data = istadv(data, len);
 
   add_new_block:
 	/* FIXME: check data.len (< 256MB) */

src/mux_h2.c

@@ -5361,16 +5361,14 @@ static size_t h2s_bck_make_req_headers(struct h2s *h2s, struct htx *htx)
 			if (len + 2 < uri.len && uri.ptr[len + 1] == '/' && uri.ptr[len + 2] == '/') {
 				/* make the uri start at the authority now */
 				scheme = ist2(uri.ptr, len);
-				uri.ptr += len + 3;
-				uri.len -= len + 3;
+				uri = istadv(uri, len + 3);
 
 				/* find the auth part of the URI */
 				auth = ist2(uri.ptr, 0);
 				while (auth.len < uri.len && auth.ptr[auth.len] != '/')
 					auth.len++;
 
-				uri.ptr += auth.len;
-				uri.len -= auth.len;
+				uri = istadv(uri, auth.len);
 			}
 		}