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.
unfortunately, the borrow checker defeated me: changing the
RwSubscription file form a Ref to a RefMut turned into borrow checker
errors in the impl of the poll_oneoff trait method.
This implementation makes an end run by having Poll hold onto the table
and fd, and borrow the file at the site of use, rather than try to own
the RefMut. I have no idea why this convinces the borrow checker that
anything is different, but it does and I need to get this PR done and
I don't think comprimising on this internal abstraction is worth
fighting against
