Use long path names on windows

This PR addresses #259 by appending the prefix needed on windows to allow paths with up to 32,767 characters.
The Microsoft page you linked to use backslashes everywhere. It also has this somewhat cryptic note:

> **Note**  File I/O functions in the Windows API convert `"/"` to `"\"` as part of converting the name to an NT-style name, except when using the `"\\?\"` prefix as detailed in the following sections.

To me, this sounds like only the backslash version should work.