Implement Wasmtime's new API as designed by RFC 11. This is quite a large commit which has had lots of discussion externally, so for more information it's best to read the RFC thread and the PR thread.
On Windows, `metadata` computes only partial metadata results, which don't
include what WASI needs for the `inode` field in `readdir` results. cap-std
has a `full_metadata` function which is able to include this extra
information, however it has more strict access requirements, so it sometimes
fails even when plain `metadata` would succeed.
Make WASI's `readdir` silently skip over files that can't be accessed by
`full_metadata`. These files wouldn't be openable in any other way by
WASI programs, so the only benefit of listing them would be to
let applications know that they exist. This allows it to avoid failing
and avoid returning bogus results.
This is part of a fix for bytecodealliance/cap-std#169.
