Draft of WASI Preview 2 support, featuring `wasi-sockets` #1061

Job	Run time
wasi-headers test	0s
Rustfmt	7s
Build libc (ubuntu-latest, 10.0.0)	1m 31s
Build libc (macos-latest, 10.0.0)	3m 36s
Build libc (windows-latest, 10.0.0)	2m 27s
Build libc (ubuntu-latest, 16.0.0, x86_64-linux-gnu-ubuntu-18.04)	2m 22s
Build libc (macos-latest, 15.0.7, x86_64-apple-darwin21.0)	4m 37s
Build libc (windows-latest, 16.0.0)	3m 10s
	17m 50s

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