Instead of an empty crate, give a compile error when not on Windows

[jyn514]

Previously, programs trying to use winapi on other targets would see
errors like this:

failed to resolve: could not find um in winapi
... many other related errors ...

Now, they'll see errors like this:

  --- stderr
  error: winapi is only supported on Windows platforms
  help: if your application is multi-platform, use `[target.'cfg(windows)'.dependencies] winapi = "..."`
  help: if your application is only supported on Windows, use `--target x86_64-pc-windows-gnu` or some other windows platform

This is a breaking change. The reason for this change is two-fold:

1. For programs intending to use winapi, it gives them a much better
   error message telling them how to fix the error.
2. For programs that don't need winapi, it forces them to remove it from
   dependencies, decreasing their compile times.

[jyn514]

CI is failing because it's still building on linux even though I told it not to, I think because it's using the CI file from the 0.3 branch instead of my PR.

[MaulingMonkey]

This would be a breaking change - I believe this should target the dev branch instead of the 0.3 branch?

[jyn514]

@retep998 what are your thoughts on this? I'm ok with closing it if you don't want it, but I do think it's a good change.