[Unison-hackers] Experimental Unicode support for Windows

[Jerome Vouillon]

I have just implemented some experimental Unicode support for
Windows.  This is not activated by default yet (I need to find
a way to make it a preference option).  But it can be actived by
editing file src/system.ml, replacing the line:
  include System_generic
by:
  include System_win
and recompiling.
Also, the directive "unicode" has to be set to true in the profile.

Testers are welcome.

The changes are quite intrusive, so I hope I have not broken too much
stuff in the process...

-- Jerome