message: minor type change for mmap vector iterator
diff: @@ -203,7 +203,7 @@ public:
  using value_type = T;
  using pointer = value_type *;
  using reference = value_type &;
  - using difference_type = long;
  + using difference_type = size_type;
  public:
  @@ -306,7 +306,7 @@ public:
  using value_type = T;
  using pointer = value_type *;
  using reference = value_type &;
  - using difference_type = long;
  + using difference_type = size_type;
  public:
  @@ -431,7 +431,7 @@ public:
  using value_type = T;
  using pointer = value_type *;
  using reference = value_type &;
  - using difference_type = long;
  + using difference_type = size_type;
  public:
  @@ -563,7 +563,7 @@ public:
  using value_type = T;
  using pointer = value_type *;
  using reference = value_type &;
  - using difference_type = long;
  + using difference_type = size_type;
  public:
  