tokio-fs: Add async versions of most of std::fs

[griff]

tokio-fs was missing most of the the functions defined in std::fs so this pull request simple adds most of them with implementations based on metadata.

I have added each function as its own commit so that the commits are very simple and easy to read.

[carllerche]

Excellent job 👍. I will schedule time to review this today.

[carllerche]

Looks great. Unfortunately, Tokio still needs to support slightly older Rust versions. Would you mind updating the doc tests to avoid using fn main() -> Result ... . Thanks!

[lnicola]

These should be exposed from tokio::fs as well. See #479, same for #488.

[griff]

@carllerche I have updated the read_dir doc test to not use fn main() -> Result ... and also got the tests to actually work.

@lnicola the functions are now exported under tokio::fs. But #488 is not relevant in that regard since it simply implements existing traits found in std::os on tokio_fs::file::File

[lnicola]

Missing self::, maybe? This failed on AppVeyor.

[griff]

It was two missing self::

[carllerche]

@griff 👍 Just need to get it to pass on Appveyor. @lnicola had a thought as to why it isn't passing above.

[griff]

@carllerche @lnicola I have fixed the missing self:: and rebased the commits so they are up to date with master

[carllerche]

👍 Great, thanks!