feat(maitake-sync): add `spin::RwLock` (#472) #1325

Job	Run time
changed_paths	4s
cargo test (host)	4m 47s
rustfmt	16s
cargo check (host)	3m 43s
clippy	2m 52s
Loom tests (cordyceps)	1m 47s
build boot image (x86_64)	3m 18s
Miri tests (codyceps)	2m 6s
docs	4m 20s
Loom tests (maitake)	4m 21s
Tests (maitake, no-default-features)	1m 45s
cargo test (cross x64)	4m 7s
Loom tests (mycelium-util)	53s
	34m 19s

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