This implementation currently uses a mix of POSIX-style APIs (provided by
`wasi-libc` via the `libc` crate) and WASIp2-native APIs (provided by the `wasi`
crate).

Alternatively, we could implement `Selector` using only POSIX APIs,
e.g. `poll(2)`.  However, that would add an extra layer of abstraction to
support and debug, as well as make it impossible to support polling
`wasi:io/poll/pollable` objects which cannot be represented as POSIX file
descriptors (e.g. timer events, DNS queries, HTTP requests, etc.).

Another approach would be to use _only_ the WASIp2 APIs and bypass `wasi-libc`
entirely.  However, that would break interoperability with both Rust `std` and
e.g. C libraries which expect to work with file descriptors.

Since `wasi-libc` does not yet provide a public API for converting between file
descriptors and WASIp2 resource handles, we currently use a non-public API (see
the `netc` module below) to do so.  Once
WebAssembly/wasi-libc#542 is addressed, we'll be able
to switch to a public API.

I've tested this end-to-end using https://github.com/dicej/wasi-sockets-tests,
which includes smoke tests for `mio`, `tokio`, `tokio-postgres`, etc.

Signed-off-by: Joel Dice <[email protected]>

// TODO tokio-rs#1: Add a public, WASIp2-only API for registering
// `wasi::io::poll::Pollable`s directly (i.e. those which do not correspond to
// any `wasi-libc` file descriptor, such as `wasi:http` requests).
//
// TODO tokio-rs#2: Add support for binding, listening, and accepting.  This would
// involve adding cases for `TCP_SOCKET_STATE_UNBOUND`,
// `TCP_SOCKET_STATE_BOUND`, and `TCP_SOCKET_STATE_LISTENING` to the `match`
// statements in `Selector::select`.
//
// TODO tokio-rs#3: Add support for UDP sockets.  This would involve adding cases for
// the `UDP_SOCKET_STATE_*` tags to the `match` statements in
// `Selector::select`.

Signed-off-by: Joel Dice <[email protected]>