std::io::fs::Path::is_dir incorrectly claims it returns false for links

[lilyball]

The documentation for is_dir in the Path impl in std::io::fs incorrectly claims that it returns false for links. It's actually implemented using stat(), which means that it will test the pointed-to file for a link.

Furthermore, .is_file() doesn't even document whether it tries to look through links.

I expect both of these files should be using stat(), so their implementations are correct, but the documentation needs to be updated to explicitly state that it will test the pointed-to file for a link.

While we're at it, .exists() says it "will return true if the path points to either a directory or a file". This can be expected to be read as "regular file", but it returns true for all file kinds, not just directories and regular files. Specifically, it just tests if stat() returns true. Incidentally, this means it will in fact return false if the path represents a symlink that does exist but points to a path that doesn't. This should also be documented.

Also, as long as we're exposing a .stat() convenience here, we should probably expose an .lstat() convenience method as well.

And finally, these methods all reference conditions (specifically, saying they don't raise conditions). That bit needs to be deleted outright.

[aturon]

This should be closed via PR 13750. /cc @alexcrichton

[alexcrichton]

Hurray!