Replace dodgy pointer operations

[yvt]

Makes the code compliant with the current aliasing model. Fixes #9.

Remaining Issues

• [x] Running MIRI in CI takes too long (4 hours in local run)
• [x] ~~Test failure in cargo +nightly miri test -p rlsf --test global nonexistent~~
  • Limitation in Stacked Borrow: https://github.com/rust-lang/miri/issues/2104
  • #18

Limitations

• Not compliant with strict provenance yet (#12) and causes Miri to generate integer-to-pointer cast warnings (Strict Provenance API requires MSRV >= 1.84)