Support fd_fdstat_get and fd_renumber on stdin/stdout/stderr (#631)

* Support fd_fdstat_get on stdin/stdout/stderr.

Add a routine for obtaining an `OsFile` containing a file descriptor for
stdin/stdout/stderr so that we can do fd_fdstat_get on them.

* Add a testcase for fd_fdstat_get etc. on stdin etc.

* Don't dup file descriptors in fd_renumber.

* Fix compilation on macOS

* Rename OsFile to OsHandle

This commits renames `OsFile` to `OsHandle` which seems to make
more sense semantically as it is permitted to hold a valid OS handle
to OS entities other than simply file/dir (e.g., socket, stream, etc.).
As such, this commit also renames methods on `Descriptor` struct
from `as_actual_file` to `as_file` as this in reality does pertain
ops on FS entities such as files/dirs, and `as_file` to `as_os_handle`
as in this case it can be anything, from file, through a socket, to
a stream.

* Fix compilation on Linux

* Introduce `OsHandleRef` for borrowing OS resources.

To prevent a `ManuallyDrop<OsHandleRef>` from outliving the resource it
holds on to, create an `OsHandleRef` class parameterized on the lifetime
of the `Descriptor`.

* Fix scoping to pub-priv and backport to snapshot_0

crates/wasi-common/src/sys/unix/linux/osfile.rs

@@ -1,32 +0,0 @@
-use std::fs;
-use std::ops::{Deref, DerefMut};
-use std::os::unix::prelude::{AsRawFd, RawFd};
-
-#[derive(Debug)]
-pub(crate) struct OsFile(fs::File);
-
-impl From<fs::File> for OsFile {
-    fn from(file: fs::File) -> Self {
-        Self(file)
-    }
-}
-
-impl AsRawFd for OsFile {
-    fn as_raw_fd(&self) -> RawFd {
-        self.0.as_raw_fd()
-    }
-}
-
-impl Deref for OsFile {
-    type Target = fs::File;
-
-    fn deref(&self) -> &Self::Target {
-        &self.0
-    }
-}
-
-impl DerefMut for OsFile {
-    fn deref_mut(&mut self) -> &mut Self::Target {
-        &mut self.0
-    }
-}