chore(maitake): use stdlib `pin!` and `poll_fn` #1394

Job	Run time
changed_paths	5s
cargo check (host)	4m 1s
Loom tests (cordyceps)	1m 37s
clippy	3m 15s
build boot image (x86_64)	3m 27s
Miri tests (cordyceps)	2m 13s
docs	4m 45s
Loom tests (maitake)	57s
Miri tests (maitake)	1m 14s
Tests (maitake, no-default-features)	1m 48s
rustfmt	27s
cargo test (host)	4m 14s
cargo test (cross x64)	4m 27s
Loom tests (mycelium-util)	1m 3s
all_systems_go	0s
	33m 33s

Provide feedback