[OT] Improve serialize syntax

For some definition of improvement...

src/hb-open-type-private.hh

@@ -600,9 +600,11 @@ struct GenericOffsetTo : OffsetType
     return StructAtOffset<Type> (base, offset);
   }
 
-  inline void set_offset (void *base, void *obj)
+  inline Type& serialize (hb_serialize_context_t *c, void *base)
   {
-    this->set ((char *) obj - (char *) base);
+    Type *t = (Type *) c->head;
+    this->set ((char *) t - (char *) base);
+    return *t;
   }
 
   inline bool sanitize (hb_sanitize_context_t *c, void *base) {

src/hb-ot-layout-gsub-table.hh

@@ -79,9 +79,8 @@ struct SingleSubstFormat1
   {
     TRACE_SERIALIZE ();
     if (unlikely (!c->extend_min (*this))) return TRACE_RETURN (false);
+    if (unlikely (!coverage.serialize (c, this).serialize (c, glyphs, num_glyphs))) return TRACE_RETURN (false);
     deltaGlyphID.set (delta);
-    coverage.set_offset (this, c->head);
-    if (unlikely (!(this+coverage).serialize (c, glyphs, num_glyphs))) return TRACE_RETURN (false);
     return TRACE_RETURN (true);
   }