Fix issue with fuzz test

Previously an implementation could never call on_begin_number or
on_end_number and still pass

src/callbacks.h

@@ -1,7 +1,11 @@
 #pragma once
 
-#include "weaseljson.h"
+#include <cstdint>
 #include <cstdio>
+#include <string>
+#include <vector>
+
+#include "weaseljson.h"
 
 inline Callbacks printCallbacks() {
   Callbacks result;
@@ -42,3 +46,96 @@ inline Callbacks noopCallbacks() {
   result.on_null_literal = +[](void *) {};
   return result;
 }
+
+struct SerializeState {
+  bool isKey = false;
+  struct Cursor {
+    int64_t index;
+    bool isObject;
+  };
+  std::string result;
+  void on_begin_value() {
+    if (!stack.empty()) {
+      auto &back = stack.back();
+      if (back.isObject && back.index % 2 == 0 && back.index > 0) {
+        result.append(",");
+      }
+      if (back.isObject && back.index % 2 == 1 && back.index > 0) {
+        result.append(":");
+      }
+      if (!back.isObject && back.index > 0) {
+        result.append(",");
+      }
+      ++back.index;
+    }
+  }
+  std::vector<Cursor> stack;
+};
+
+inline Callbacks serializeCallbacks() {
+  Callbacks result;
+  result.on_begin_object = +[](void *p) {
+    auto *state = (SerializeState *)p;
+    state->on_begin_value();
+    state->stack.push_back({0, true});
+    state->result.append("{");
+  };
+  result.on_end_object = +[](void *p) {
+    auto *state = (SerializeState *)p;
+    state->stack.pop_back();
+    state->result.append("}");
+  };
+  result.on_begin_string = +[](void *p) {
+    auto *state = (SerializeState *)p;
+    state->on_begin_value();
+    state->result.append("<");
+  };
+  result.on_string_data = +[](void *p, const char *buf, int len) {
+    auto *state = (SerializeState *)p;
+    state->result.append(std::string(buf, len));
+  };
+  result.on_end_string = +[](void *p) {
+    auto *state = (SerializeState *)p;
+    state->result.append(">");
+  };
+  result.on_begin_array = +[](void *p) {
+    auto *state = (SerializeState *)p;
+    state->on_begin_value();
+    state->stack.push_back({0, false});
+    state->result.append("[");
+  };
+  result.on_end_array = +[](void *p) {
+    auto *state = (SerializeState *)p;
+    state->stack.pop_back();
+    state->result.append("]");
+  };
+  result.on_begin_number = +[](void *p) {
+    auto *state = (SerializeState *)p;
+    state->on_begin_value();
+    state->result.append("(");
+  };
+  result.on_number_data = +[](void *p, const char *buf, int len) {
+    auto *state = (SerializeState *)p;
+    state->result.append(std::string(buf, len));
+  };
+  result.on_end_number = +[](void *p) {
+    auto *state = (SerializeState *)p;
+    state->result.append(")");
+  };
+  result.on_true_literal = +[](void *p) {
+    auto *state = (SerializeState *)p;
+    state->on_begin_value();
+    state->result.append("true");
+  };
+  result.on_false_literal = +[](void *p) {
+    auto *state = (SerializeState *)p;
+    state->on_begin_value();
+    state->result.append("false");
+  };
+  result.on_null_literal = +[](void *p) {
+    auto *state = (SerializeState *)p;
+    state->on_begin_value();
+    state->result.append("null");
+  };
+  return result;
+}