std: Deprecate std::old_io::fs

This commit deprecates the majority of std::old_io::fs in favor of std::fs and
its new functionality. Some functions remain non-deprecated but are now behind a
feature gate called `old_fs`. These functions will be deprecated once
suitable replacements have been implemented.

The compiler has been migrated to new `std::fs` and `std::path` APIs where
appropriate as part of this change.

src/libstd/sys/unix/ext.rs

@@ -54,6 +54,7 @@ pub trait AsRawFd {
     fn as_raw_fd(&self) -> Fd;
 }
 
+#[allow(deprecated)]
 impl AsRawFd for old_io::fs::File {
     fn as_raw_fd(&self) -> Fd {
         self.as_inner().fd()