Misc yanix fixes (#715)

* Correctly handle possibly misaligned pointers in readdir

This reapplies #615, which was inadvertently reverted.

* Tidy up unneeded `self::` qualifiers.

* Make Dir's contents private.

Also remove the `unsafe` from `impl_iter`. With `Dir`'s field being
private, we can rely on the pointer being only what we've assigned to
it.

* Make `poll`'s timeout argument a `libc::c_int`.

This clarifies that there are no subsequent conversions before calling the
underlying libc API.

* Use clock_gettime instead of clock_getres to get the time.

* Mark FileType::from_raw as safe.

It handles unknown values, so it can be marked safe.

crates/wasi-common/src/host.rs

@@ -63,12 +63,12 @@ impl Dirent {
 
         let sys_dirent = raw.as_mut_ptr() as *mut __wasi_dirent_t;
         unsafe {
-            *sys_dirent = __wasi_dirent_t {
+            sys_dirent.write_unaligned(__wasi_dirent_t {
                 d_namlen: namlen.try_into()?,
                 d_ino: self.ino,
                 d_next: self.cookie,
                 d_type: self.ftype.to_wasi(),
-            };
+            });
         }
 
         let sys_name = unsafe { sys_dirent.offset(1) as *mut u8 };