Fix miscompilation in transmute_unchecked introduced by bug in LLVM

[zheland]

Currently some casts from byte primitives to two element tuple lead to miscompilation on release builds on Rust >=1.70.0:

• castaway::cast!(123_u8, (u8, u8)) unexpectedly returns Ok(...) that leads to UB.
• castaway::cast!(false, (bool, u16)) leads to SIGILL: illegal instruction runtime error.

Upstream issues:

• Rust: Miscompilation on release profile when std::ptr::read is used to cast byte primitive to some tuples in unreachable code paths rust-lang/rust#127286
• LLVM: [Mem2Reg] Incorrect poison propagation llvm/llvm-project#97702

I suggest considering adding a safe "workaround" to fix the issue in this crate without having to wait for the upstream fixes. This way we will have this fixed in older Rust versions as well.

This PR adds size eq assert to transmute_unchecked. This workaround was found while preparing an MRE for an upstream issue.

Checked locally with cargo test --release for Rust 1.38, 1.68.0, 1.69.0, 1.70.0, 1.71.0, 1.72.0, stable, beta, nightly. Generated assembly for other tests cases for the release build seems the same (checks and casts are optimized away).

Btw: it might also be a good idea to run tests in --release mode as well since the crate relies heavily on optimizing the casts to zero-cost.

[sagebind]

Thanks for finding this and opening a PR! I will push out a patch release with this ASAP.