Refactor use of Refs and RefMuts in wasi-common (#1412)

* Refactor use of Refs and RefMuts in wasi-common

This commit refactors the use of `Ref`s and `RefMut`s in `wasi-common`.
Now, `Entry` is stored behind an `Rc` inside the `EntryTable`. The `Entry`
itself on the other hand now stores rights behind a `RefCell` and the
descriptor as `Rc<RefCell<..>>` combo to enable easy reference tracking
and interior mutability which is required down the line in a couple of
syscalls. In essence, this implies that we no longer have need for
mutable accessor to `Entry` from `WasiCtx`, and so all related methods
go away (`get_entry_mut`, etc.).

While here, I've also simplified handling and aggregating of rights on
the `Entry` object. Instead of storing base and inheriting rights as
separate fields, they are now aggregated into one struct `EntryRights`
which features convenient constructors for each possible combination; i.e.,
when only base rights are set, or both base and inheriting are set, or
both are left as empty. Since we do need to be able to mutate those
rights down the line in `fdstat_set_rights` syscall, this object
is kept behind a `RefCell` (note no `Rc` since we don't need to pass it
around anywhere).

The descriptor field in `Entry` is now kept behind `Rc<RefCell<..>>` combo
since we not only need to mutate it down the line, but we also need to
be able to pass it around (as part of the machinery making `poll_oneoff`
work).

I've also removed `as_file` and `try_clone` methods on `Descriptor` struct
since they were adding more noise than necessary, and making them work
with `Rc` was unnecessarily complicated.

Finally, I've converted the `get_dir_from_os_handle` function into a
method attached to the `OsHandle` itself, called `dir_stream`. IMHO,
it makes more sense to have it there directly as a method than as a separate
function.

* Use Cell for types that are Copy

crates/wasi-common/src/sys/unix/bsd/oshandle.rs

@@ -1,4 +1,5 @@
-use std::cell::RefCell;
+use crate::wasi::Result;
+use std::cell::{RefCell, RefMut};
 use std::fs;
 use std::ops::Deref;
 use std::os::unix::prelude::{AsRawFd, RawFd};
@@ -6,7 +7,7 @@ use yanix::dir::Dir;
 
 #[derive(Debug)]
 pub(crate) struct OsHandle {
-    pub(crate) file: fs::File,
+    file: fs::File,
     // In case that this `OsHandle` actually refers to a directory,
     // when the client makes a `fd_readdir` syscall on this descriptor,
     // we will need to cache the `libc::DIR` pointer manually in order
@@ -18,7 +19,26 @@ pub(crate) struct OsHandle {
     //   > of the DIR pointer, dirp, from which they are derived.
     //   > If the directory is closed and then reopened, prior values
     //   > returned by telldir() will no longer be valid.
-    pub(crate) dir: RefCell<Option<Dir>>,
+    dir: RefCell<Option<Dir>>,
+}
+
+impl OsHandle {
+    pub(crate) fn dir_stream(&self) -> Result<RefMut<Dir>> {
+        if self.dir.borrow().is_none() {
+            // We need to duplicate the fd, because `opendir(3)`:
+            //     Upon successful return from fdopendir(), the file descriptor is under
+            //     control of the system, and if any attempt is made to close the file
+            //     descriptor, or to modify the state of the associated description other
+            //     than by means of closedir(), readdir(), readdir_r(), or rewinddir(),
+            //     the behaviour is undefined.
+            let fd = self.file.try_clone()?;
+            let d = Dir::from(fd)?;
+            *self.dir.borrow_mut() = Some(d);
+        }
+        Ok(RefMut::map(self.dir.borrow_mut(), |dir| {
+            dir.as_mut().unwrap()
+        }))
+    }
 }
 
 impl From<fs::File> for OsHandle {