Add support for WASI 0.2

[yoshuawuyts]

The WASI 0.2 target will reach tier 2 in Rust 1.82. This crate supports WASI 0.1, and it would be neat if it could support WASI 0.2 as well.

I looked into it briefly, and I believe this depends on rust-random/getrandom#499 landing first since that's used internally. Landing this in turn should enable us to update the polling crate, which uses fastrand in its test suite.

[taiki-e]

Since it has a valid std::time::Instant, this crate does nothing special with WASI.

[yoshuawuyts]

Yeah, that seems about right. When I first filed this issue support for that wasn't in the stdlib, but happy to close it now!