[wasi-common] Clean up the use of mutable Entry (#1395)

* Clean up the use of mutable Entry

Until now, several syscalls including `fd_pwrite` etc. were relying on
mutating `&mut Entry` by mutating its inner file handle. This is
unnecessary in almost all cases since all methods mutating `std::fs::File`
in Rust's libstd are also implemented for `&std::fs::File`.

While here, I've also modified `OsHandle` in BSD to include `RefCell<Option<Dir>>`
rather than `Option<Mutex<Dir>>` as was until now. While `RefCell`
could easily be replaced with `RefCell`, since going multithreading
will require a lot of (probably even) conceptual changes to `wasi-common`,
I thought it'd be best not to mix single- with multithreading contexts
and swap all places at once when it comes to it.

I've also had to make some modifications to virtual FS which mainly
swapped mutability for interior mutability in places.

* Use one-liners wherever convenient

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

@@ -1,7 +1,7 @@
+use std::cell::RefCell;
 use std::fs;
-use std::ops::{Deref, DerefMut};
+use std::ops::Deref;
 use std::os::unix::prelude::{AsRawFd, RawFd};
-use std::sync::Mutex;
 use yanix::dir::Dir;
 
 #[derive(Debug)]
@@ -18,12 +18,15 @@ 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: Option<Mutex<Dir>>,
+    pub(crate) dir: RefCell<Option<Dir>>,
 }
 
 impl From<fs::File> for OsHandle {
     fn from(file: fs::File) -> Self {
-        Self { file, dir: None }
+        Self {
+            file,
+            dir: RefCell::new(None),
+        }
     }
 }
 
@@ -40,9 +43,3 @@ impl Deref for OsHandle {
         &self.file
     }
 }
-
-impl DerefMut for OsHandle {
-    fn deref_mut(&mut self) -> &mut Self::Target {
-        &mut self.file
-    }
-}