text: Title: Implement saving to Facebook format
  Body: We currently support reading FastText models from Facebook's format. The [gensim.models._fasttext_bin](https://github.com/RaRe-Technologies/gensim/blob/develop/gensim/models/_fasttext_bin.py) does this.
  
  This enables people to use gensim with a model that was trained using Facebook's binaries.
  
  Sometimes, people want things to work the other way: they start with gensim, train a model, and then want to save it to Facebook's format.
  
  For this ticket, you will implement a `save(model, fout)` function that accepts a FastText object and saves it to a file stream in a Facebook-compatible format. It will essentially reverse the effects of the [load](https://github.com/RaRe-Technologies/gensim/blob/f89808d52d0250e4e4bbab2293980f8f4d3989b9/gensim/models/_fasttext_bin.py#L291) function. 