Support long file names on Windows

[DemiMarie]

Windows does not support file names of >255 UTF-16 characters, unless
the path is prefixed with \\?\ or \??\. Both of those, however,
supress normalization. So Rust must do that normalization itself. This
commit makes the standard library do just that, by calling the Windows
API function GetFullPathNameW().

This implements rust-lang/rfcs#2188.

This also fixes a bug where a path beginning with \??\ on Windows was incorrectly marked as not being a verbatim path. That it is such was confirmed by reading the .NET CoreFX sources.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @sfackler (or someone else) soon.

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[retep998]

One case that your PR does not seem to handle is device paths starting with \\.\. They are equivalent to \\?\ paths except for the fact that they don't have to be normalized. After normalizing them, swapping out the \\.\ for \\?\ should have the right result.

Also, there is no way I'd accept this being merged without an extensive set of tests covering every possible style of path. Because crater won't be able to effectively test the impact of this change, tests are even more critically important.

[shepmaster]

Heya @DemiMarie — it's been over 7 days since we've heard from you. Will you have any time to address the request for more tests?

[shepmaster]

Thanks for the PR, @DemiMarie! Unfortunately, we haven't heard from you in a few weeks. I'm going to close this PR for now to keep things tidy, but please feel free to re-open it once you've had a chance to address some of the feedback!