message: Fix build on MSVC 32-bit
diff: @@ -94,13 +94,13 @@ public:
  paths.emplace_back(ptr);
  }
  - long credit_for(entity& x) {
  + size_t credit_for(entity& x) {
  auto pred = [&](outbound_path* ptr) {
  return ptr->hdl == &x;
  };
  auto i = std::find_if(paths.begin(), paths.end(), pred);
  CAF_REQUIRE(i != paths.end());
  - return (*i)->open_credit;
  + return static_cast<size_t>((*i)->open_credit);
  }
  void new_round(int num, bool force_emit) {
  @@ -274,7 +274,7 @@ receive_checker<F> operator<<(receive_checker<F> xs, not_empty_t) {
  #define AFTER
  -#define HAS ; auto CONCAT(amount, __LINE__) =
  +#define HAS ; size_t CONCAT(amount, __LINE__) =
  #define CREDIT ;
  @@ -299,7 +299,7 @@ receive_checker<F> operator<<(receive_checker<F> xs, not_empty_t) {
  #define FOR \
  struct CONCAT(for_helper_, __LINE__) { \
  entity& who; \
  - long amount; \
  + size_t amount; \
  void operator<<(entity& x) const { \
  CAF_CHECK_EQUAL(who.credit_for(x), amount); \
  } \
  