* Implemented `reduced_windows` argument for Word2Vec.

Co-Authored-By: Mathis Demay <mathis.demay.md@gmail.com>

* Improve the way `reduced_windows` is passed around and used.

* Renamed `reduced_windows` to `shrink_windows`.

* Removed `shrink_windows` argument from `Word2Vec.train`.

* Aesthetic fix.

* Fixed old word2vec models' reloading.

* Fixed undue docstring.

* Added `shrink_windows` argument to Doc2Vec.

* Added `shrink_windows` argument to FastText.

* Fixed and optimized `shrink_windows` backend use.

* `c.reduced_windows[:] = 0` syntax is not supported;
  as a consequence, all zero-value assignments due to
  `shrink_windows=False` have been rewritten as `for`
  loops.
* Those changes have now been tested (cf. next commit).
* NOTE: another way to proceed could be to initialize
  the `reduced_windows` array with zeros as values; it
  would then only be altered when `shrink_windows=True`.

* Added tests for `shrink_windows=False` in Word2Vec-based models.

* Added docstring mentions of `shrink_window` being experimental.

* Rolled back some purely aesthetic changes.

Co-authored-by: Paul Andrey <paul.andrey@hotmail.fr>