implement wasi-filesystem::readdir and related functions (bytecodea…

…lliance#45)

* implement `wasi-filesystem::readdir` and related functions

This adds a `directory_list` test and provides the required host implementation.

I've also added a file length check to the `file_read` test, just to cover a bit
more of the API.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

* fix memory corruption in `fd_readdir` polyfill

We were copying `name.len() * 256` bytes instead of just `name.len()` bytes,
which was overwriting other parts of `State` and causing untold havoc.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

* check type of entry in `Table::delete`

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

src/lib.rs

@@ -911,8 +911,8 @@ pub unsafe extern "C" fn fd_readdir(
                 state.dirent_cache.cookie.set(cookie - 1);
                 state.dirent_cache.cached_dirent.set(dirent);
                 std::ptr::copy(
-                    name.as_ptr().cast(),
-                    (*state.dirent_cache.path_data.get()).as_mut_ptr(),
+                    name.as_ptr().cast::<u8>(),
+                    (*state.dirent_cache.path_data.get()).as_mut_ptr() as *mut u8,
                     name.len(),
                 );
                 break;