todo: audit code against the &Header problem

[pnkfelix]

rust-lang/unsafe-code-guidelines#256 discusses issues with how one might choose to encode the common C pattern of transmuting back and forth from a pointer to a struct and a pointer to a prefix of that struct.

glibc setjmp inherently does this.

I don't think its actually a real issue for this crate, because the JmpBufFields and SigJmpBufFields should really be extern types, not accessible on the Rust side of things.

But at the same time, I am passing a &'a _ around, in order to prevent people from using the reference outside of its scope. And so I just want to double check that my usage here is allowed.

(unfortunately I don't expect miri to be any help for us checking this, since it doesn't support foreign code nor inline assembly. It would be pretty cool if I could get krabcake far enough along for it to make a statement one way or another about what I'm doing here.)