Extract windows_registry module into its own crate

[brson]

This crate is out of place here. Put it in another crate and make an internal dependency of this crate.

I'd suggest calling the crate msvc but we probably need to bikeshed the name further. @retep998 @alexcrichton preferences?

We'll also need to negotiate where it lives. At the least @alexcrichton will need to be a maintainer, so maybe he should just do this one.

[alexcrichton]

I'd probably go for a name like msvc-finder or something like that, I'm not sure this crate would be good enough to take a good name like msvc!

[retep998]

There's also bikeshedding on names in retep998/msvc-bunny#1

[alexcrichton]

I'm going to close this as this isn't going to happen for the 1.0 release. When it does happen, though, I'll happily deprecate the functionality that already exists!