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/test-programs/wasi-tests/src/bin/stdio.rs

@@ -0,0 +1,31 @@
+use libc;
+use std::mem::MaybeUninit;
+use std::{env, process};
+use wasi::wasi_unstable;
+use wasi_tests::wasi_wrappers::wasi_fd_fdstat_get;
+
+unsafe fn test_stdio() {
+    for fd in &[
+        wasi_unstable::STDIN_FD,
+        wasi_unstable::STDOUT_FD,
+        wasi_unstable::STDERR_FD,
+    ] {
+        let mut fdstat: wasi_unstable::FdStat = MaybeUninit::zeroed().assume_init();
+        let status = wasi_fd_fdstat_get(*fd, &mut fdstat);
+        assert_eq!(
+            status,
+            wasi_unstable::raw::__WASI_ESUCCESS,
+            "fd_fdstat_get on stdio"
+        );
+
+        assert!(
+            wasi_unstable::fd_renumber(*fd, *fd + 100).is_ok(),
+            "renumbering stdio",
+        );
+    }
+}
+
+fn main() {
+    // Run the tests.
+    unsafe { test_stdio() }
+}