[DRAFT, WINDOWS] Use platform-specific separator

Possible support for https://github.com/rspec/rspec-dev/issues/83
Can you explain this line?  Specifically:

* What is the difference between `File::SEPARATOR` and `File::ALT_SEPARATOR`?  I can't find much documentation  on why they both exist and how they differ.
* Why do you check for the existence of `File::ALT_SEPARATOR` but then not use it, using a constant `"\\\\"` instead?