once_cell is an extremely widely-used crate, so it would be very nice if
it conformed to the stricted/simplest/checkable model we have for
aliasing in Rust. To do this, we need to avoid creating a pointer from
an integer by cast or transmute. Pointers created this way can be valid,
but are a nightmare for a checker like Miri. The situation is generally
explained by these docs: https://doc.rust-lang.org/nightly/std/ptr/fn.from_exposed_addr.html

This implementation is mostly gross because all the APIs that would make
this ergonomic are behind #![feature(strict_provenance)]

…enance

This branch merges upstream's `master` branch into PR matklad#185.

I need to patch a dependency on `once_cell` that needs at least 1.13.0
to pick up the provenance fix, but you may want to actually merge this
into the PR as well.