Remove HalSurface and fix layout assumptions in AnySurface and AnyDevice

[udoprog]

Connections
#5080

Description
This avoids figuring out if #5080 is an actual problem by introducing an internal vtable for the types in question instead of using a trait object. This also affords us to stored the backend statically in the vtable, which hopefully makes figuring out which backend an AnySurface belongs to much nicer.

I've also reworked AnySurface to avoid a double-indirection, since AFAICT it only really needs to hold onto an A::Surface. So it can be directly boxed.

Checklist

• Run cargo fmt.
• Run cargo clippy.
• Run cargo xtask test to run tests.

[ErichDonGubler]

I've done a slow pass over this, and the broad strokes LGTM. It's always good to avoid "We upgraded rustc, and things broke, and it's our fault." 😅 I want to do another review pass focused on the fine-grained logic and types here, since my experience with Rust FFI is that there's often subtle bugs lurking in even the best human's code that aren't obvious. Will hopefully report back in the next day or so.

[ErichDonGubler]

praise: This is a nice simplification and fix. Thanks! 🙂

Overall, LGTM minus suggestions and nits! I'm marking as Request changes so that the issues I've found get handled, but I'm confident in the overall thrust of this PR. On that note, I'm going to prompt for feedback from @Wumpf and/or @cwfitzgerald, since while I've done quite a bit of FFI/pointer-slinging in Rust before, I'm not as familiar with native platform domain/API for surfaces and devices.

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_{I used Conventional Comments in this review! I hope they help with clarity and tone. 🙂}

[udoprog]

Thanks for taking a look @ErichDonGubler!

I'll rebase the change once it has been looked over by all parties who might find it objectionable and the outstanding questions have been resolved.

[ErichDonGubler]

LGTM now! Planning on waiting to merge this for a day or so, to give @jimblandy or @cwfitzgerald a chance to look over the pings I've given them specifically. I don't see a strong impetus to wait longer than that, though.

[udoprog]

All right, I've rebased the change to try and get CI green so you're unblocked once you make a decision.

I've also included this minor nit, because it doesn't make much sense to format one as a tuple and omit useful information in the other.