wasi-c2: implement a synchronous poll_oneoff in terms of yanix

doesnt work on unix yet! also breaks all the rules about using the
cap-std family instead of rawfds! but this is cool and im happy with it

crates/wasi-c2/Cargo.toml

@@ -32,6 +32,7 @@ cap-rand = "0.9"
 fs-set-times = "0.2.2"
 cfg-if = "1"
 bitflags = "1.2"
+yanix = "0.22"
 
 [badges]
 maintenance = { status = "actively-developed" }