111.28.00

- Implemented `Int.gcd` using binary GCD in C, for improved performance.
- Added `Bin_io_utils.Serialized`, which stores a value in memory as its
  bin-io representation.

    Writing such a value just blits the value.
- Moved `Text_block` from `Core_extended` into `Textutils`.
- Added modules `Hashtbl2` and `Hashtbl2_pair`.