Remove Windows XP support

[newpavlov]

Closes#65.

[josephlr]

I'm totally down with removing XP support, but I'm not sure about switching over the implementation for Windows 7,8,10

Initially, I thought it would be fine, but the fact that Go and BoringSSL didnt switch means this probably should have a second look. I'll review the logic in those threads and try to see if I can find any (external or internal) reasoning for not switching.

I suspect that it doesn't matter, in which case we should switch implementations, but I want to be sure.

[josephlr]

@newpavlov I kept the content of this PR the same, but redid the commit ordering. That way, when this PR is merged, windows.rs will keep the windows_uwp.rs git blame (instead having this PR appear like a change to windows.rs).

[josephlr]

Approved, see my comment in the bug for why I think switching is fine.