[review] keyring/credentials on Windows

Hello all,

After a year, I'm finally restarting development on my cross-platform keyring library. I've just completed the windows backend. (osx and linux already have working backends)

Since Windows is not my primary platform, and because I've never interacted with the windows api before, it would be great to have some eyes on my code (first time using pointers and unsafe).

If you're interested in the rest of the library also, feel free to poke around. I have plans to tidy it up and have better docs; a short list is in the issues. General feedback is definitely welcome. I would like this to be useful to the rust community.