Validate correct string data in fuzz test

src/callbacks.h

@@ -7,8 +7,8 @@
 
 #include "weaseljson.h"
 
-inline Callbacks printCallbacks() {
-  Callbacks result;
+inline WeaselJsonCallbacks printCallbacks() {
+  WeaselJsonCallbacks result;
   result.on_begin_object = +[](void *) { puts("on_begin_object"); };
   result.on_end_object = +[](void *) { puts("on_end_object"); };
   result.on_begin_string = +[](void *) { puts("on_begin_string"); };
@@ -29,8 +29,8 @@ inline Callbacks printCallbacks() {
   return result;
 }
 
-inline Callbacks noopCallbacks() {
-  Callbacks result;
+inline WeaselJsonCallbacks noopCallbacks() {
+  WeaselJsonCallbacks result;
   result.on_begin_object = +[](void *) {};
   result.on_end_object = +[](void *) {};
   result.on_begin_string = +[](void *) {};
@@ -72,8 +72,8 @@ struct SerializeState {
   std::vector<Cursor> stack;
 };
 
-inline Callbacks serializeCallbacks() {
-  Callbacks result;
+inline WeaselJsonCallbacks serializeCallbacks() {
+  WeaselJsonCallbacks result;
   result.on_begin_object = +[](void *p) {
     auto *state = (SerializeState *)p;
     state->on_begin_value();