Make `UserData` pointer-packing actually sound #857

Job	Run time
Build aarch64-apple-darwin (macOS-12, aarch64-apple-darwin)	2m 15s
cargo-deny	19s
Lint	4m 5s
Build (Release) (aarch64-linux-android)	2m 56s
Package	1m 51s
Test (Debug) (ubuntu-22.04, x86_64-unknown-linux-gnu)	12m 5s
Test (Debug) (ubuntu-22.04, aarch64-unknown-linux-gnu, ghcr.io/cross-rs/aarch64-unknown-linux-gnu...	12m 40s
Test (Debug) (windows-2022, x86_64-pc-windows-msvc)	13m 4s
Test (Debug) (macOS-12, x86_64-apple-darwin)	10m 31s
	59m 46s

Provide feedback