Expose raw handles for the Poller

[notgull]

Most of the current Poller instances are wrappers around either a RawFd or a RawHandle. For these platforms, an AsRawFd or AsRawHandle implementation is implemented on Poller. Then, because I have absolutely no self control, I also went ahead and conditionally implemented AsFd and AsHandle as well on conditionally proper Rust versions through autocfg. See sunfishcode/io-lifetimes#38 for more information.

This is part of my attempt at getting smol-rs/async-io#39 to work

[zeenix]

LGTM otherwise.

[taiki-e]

This a Cargo-based project and I really don't see why we should sacrifice the readability of this project for possible users that use cargo-based projects w/o ensuring their build system is actually doing everything cargo-based project expects.

AFAIK, non-cargo build systems such as Bazel, Buck, and GN have helpers (e.g., cargo-raze, cargo-gnaw) to create manifests for that build system based on Cargo.toml and can support cargo-based projects. However, running arbitrary other rust code (autocfg) at build time may or may not be simple. Therefore, it is helpful to have reasonable defaults in case the build script is not run. (AFAIK, this was first proposed in serde-rs/serde#1593.)

I maintain several projects that sometimes receive feedback from fuchsia team (using GN, IIRC), so I usually prefer to use negative cfgs so that it works without friction in their code base. (Also, it seems some smol-rs projects are being used in fuchsia.)

[zeenix]

@taiki-e Thanks so much for indulging and educating me. Much appreciated! 👍