Expose reference external sequence API

Expose reference external sequence API

* Expose the reference external sequences API for zstdmt.
  Allows external sequences of any length, which get split when necessary.
* Reset the LDM window when the context is reset.
* Store the maximum number of LDM sequences.
* Sequence generation now returns the number of last literals.
* Fix sequence generation to not throw out the last literals when blocks of
  more than 1 MB are encountered.