Safe external statics

[madsmtm]

Some types like NSErrorUserInfoKey are really just a list of certain NSStrings that may be valid where it is used.

It would be nice to have a better way of handling these!

Something like rust-lang/lang-team#149 could help here, likewise if we had associated statics.

[madsmtm]

This is now possible with Rust 1.82, so we can do this once that's in our MSRV.