add `wasm32-wasip2` support

[dicej]

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 https://github.com/WebAssembly/wasi-libc/issues/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.