[DRAFT] Replace AsRef<Path> with PathLike

[ChrisDenton]

A minimal implementation of rust-lang/libs-team#62.

pub trait PathLike {
    /// Convert to a `Path` reference.
    fn with_path<T, F: FnOnce(&Path) -> T>(&self, f: F) -> T;
    /// Convert to the native platform representation of a path.
    fn with_native_path<T, F: FnOnce(&NativePath) -> io::Result<T>>(&self, f: F) -> io::Result<T>;
}

/// Represents a path in the OS' native format.
///
/// Use OS specific extension trait to access the inner type.
pub struct NativePath(/* private */);

To keep things simple, this PR currently only uses with_native_path within File::open. Other fs function use with_path.

r? @ghost

[ChrisDenton]

@rustbot label -S-waiting-on-review +S-waiting-on-author