Run AddressSanitizer and fix leak

[madsmtm]

Run with:

ASAN_OPTIONS=detect_leaks=1 RUSTFLAGS="-Zsanitizer=address" cargo +nightly test --target=x86_64-apple-darwin -Zbuild-std --tests -- --skip=foundation::error::tests --skip=test_weak

Notably, I incorrectly assumed that the super class' dealloc method is called in custom dealloc methods, but this is only true in Objective-C with ARC enabled!

Builds upon #273.