Be aware of alignment checking issues

We have code like the following in zerocopy as of this writing:

https://github.com/google/zerocopy/blob/27aea679379090ad7de96bf05ab935e9f6edaa56/src/lib.rs#L2323-L2331

According to https://github.com/rust-lang/rust/issues/62420, there's a small chance that this may be decided to be unsound (and it's not supported during const eval, although there are bigger problems regarding using addresses in const eval). We should figure out whether this is a risk we're comfortable with, and consider how we might mitigate it.