chore(maitake): use stdlib `pin!` and `poll_fn` #1422

Job	Run time
changed_paths	4s
rustfmt	22s
cargo check (host)	3m 49s
Loom tests (cordyceps)	1m 34s
Miri tests (cordyceps)	2m 10s
docs	4m 22s
clippy	3m 2s
Loom tests (maitake)	42s
cargo test (host)	3m 47s
cargo test (cross x64)	4m 36s
build boot image (x86_64)	3m 21s
Miri tests (maitake)	1m 5s
Tests (maitake, no-default-features)	1m 49s
all_systems_go	0s
	30m 43s

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