Fix assertion for same DB in DbGuard

[MichaReiser]

Working on #529 I ran into the issue that the assertion in DbGuard asserting that the DB is unchanged consistently fails.

The underlying problem is that NonNull::eq not only compares the pointer but also the metadata which is unreliable for trait object pointers

When comparing wide pointers, both the address and the metadata are tested for equality. However, note that comparing trait object pointers (*const dyn Trait) is unreliable: pointers to values of the same underlying type can compare inequal (because vtables are duplicated in multiple codegen units), and pointers to values of different underlying type can compare equal (since identical vtables can be deduplicated within a codegen unit).
source

This PR changes the assertion to use std::ptr::addr_eq which only compares the address without the metadata.

Fixes #536

[netlify]

✅ Deploy Preview for salsa-rs canceled.

Name	Link
🔨 Latest commit	354dc0e
🔍 Latest deploy log	https://app.netlify.com/sites/salsa-rs/deploys/66a51f2a77f0680008102189

[MichaReiser]

@nikomatsakis do you think we could merge this to mitigate the immediate panic. I know you're working on a more long-term fix, but it would unblock some short-term usage and benchmarks.

[nikomatsakis]

OK, I'm a bit confused why the metadata is different but... sounds good.