The semantics of int-to-pointer casts under strict provenance has changed. Ideally, we would update our `From<usize> for Register` implementation to use the `sptr` crate, but `sptr` does not currently have a suitable license. There is a [PR](Gankra/sptr#14) to change that, but it hasn't gotten a response in 7 months, so I don't think we can use it. Instead, this copies the implementation of `core::ptr::invalid`.

This also bumps our `thiserror` dependency, which forces Cargo to pick a newer version of `proc-macro2` that is compatible with recently nightly toolchains.

Only use nightly for miri.