Unify fd_readdir impl between *nixes (#613)

* Unify fd_readdir impl between *nixes

This commit unifies the implementation of `fd_readdir` between Linux
and BSD hosts. In particular, it re-uses the `Dirent`, `Entry`, and
`Dir` (among others) building blocks introduced recently when
`fd_readdir` was being implemented on Windows.

Notable changes:
* on BSD, wraps `readdir` syscall in an `Iterator` of the mutex-locked
  `Dir` struct
* on BSD, removes `DirStream` struct from `OsFile`; `OsFile` now holds a
  mutex to `Dir`
* makes `Dir` iterators implementation specific (Linux has its own,
  and so does BSD)

* Lock mutex once only; explain dir in OsFile

* Add more comments

crates/wasi-common/src/sys/windows/hostcalls_impl/fs.rs

@@ -218,7 +218,7 @@ fn dirent_from_path<P: AsRef<Path>>(
 // .        gets cookie = 1
 // ..       gets cookie = 2
 // other entries, in order they were returned by FindNextFileW get subsequent integers as their cookies
-pub(crate) fn fd_readdir_impl(
+pub(crate) fn fd_readdir(
     fd: &File,
     cookie: wasi::__wasi_dircookie_t,
 ) -> Result<impl Iterator<Item = Result<Dirent>>> {
@@ -257,30 +257,6 @@ pub(crate) fn fd_readdir_impl(
     Ok(iter.skip(cookie))
 }
 
-// This should actually be common code with Linux
-pub(crate) fn fd_readdir(
-    os_file: &mut OsFile,
-    mut host_buf: &mut [u8],
-    cookie: wasi::__wasi_dircookie_t,
-) -> Result<usize> {
-    let iter = fd_readdir_impl(os_file, cookie)?;
-    let mut used = 0;
-    for dirent in iter {
-        let dirent_raw = dirent?.to_wasi_raw()?;
-        let offset = dirent_raw.len();
-        if host_buf.len() < offset {
-            break;
-        } else {
-            host_buf[0..offset].copy_from_slice(&dirent_raw);
-            used += offset;
-            host_buf = &mut host_buf[offset..];
-        }
-    }
-
-    trace!("     | *buf_used={:?}", used);
-    Ok(used)
-}
-
 pub(crate) fn path_readlink(resolved: PathGet, buf: &mut [u8]) -> Result<usize> {
     use winx::file::get_file_path;