Windows support

[anishathalye]

I briefly tested Periscope on Windows, and in manual tests without tricky corner cases, it seems to work. The tests don't currently work -- the tests using testfs (which relies on afero's MemFs) seem to hang / OOM, and the tests using the real filesystem and fancy features like symlinks don't work either.

To support Windows, we need to:

• Figure out why the MemFs-based tests aren't working, and fix them
• Figure out why tests using the real filesystem aren't working, and either split them into separate files that are only used on macOS/Linux, or figure out how to adapt them to be cross-platform
• Understand any potential subtleties of Periscope on Windows/NTFS, fix the code to handle these cases, and add Windows-specific tests

(I don't use Windows, so this is a low-priority issue for me. There's a good chance this will only be supported if someone can help out with this.)