Use `expose_addr()` in `fmt::Pointer` Discussion in https://rust-lang.zulipchat.com/#narrow/stream/136281-t-lang.2Fwg-unsafe-code-guidelines/topic/Should.20.60fmt.3A.3APointer.60.20expose.20the.20argument.3F on whether or not we should do this (still undecided). CC `@RalfJung`