[WIP] Move wasip2 impl to sys/p2

[yoshuawuyts]

Following up on #127, this PR migrates the core structure of the repo to something more similar to the polling crate. It does this by creating separate impls-per-target under sys/, with a single cfg-if switching on them in the sys/mod.rs:

cfg_if::cfg_if! {
    if #[cfg(all(target_os = "wasi", target_env = "p2"))] {
        mod p2;
        use p2 as backend;
    } else {
        compile_error!("unsupported target: wstd only compiles on `wasm32-wasip2`");
    }
}

pub use backend::*;

This general approach should result in fewer inline cfgs and a better general separation of concerns. But it's somewhat unavoidable that we'll end up with different code paths for p2 and p3 since the underlying async model is different. Something will need to abstract over those differences, and typically that job lands on runtimes. Unless of course we don't want to support p3 in wstd, but if that's the case we should probably talk that over sync sometime.

I'm putting this up as a draft because I generated this with an agent and haven't yet gone over this in detail. The changes are mostly mechanical though, so I don't expect much to be off. I prefer to review changes like these via the github interface, which is why I'm submitting this as-is. But like: please don't feel obliged to review this right now!

[yoshuawuyts]

Yep, I'm already noticing some comments and the like have been lost in the transition. This is going to need more work.