Kernel: Adjustments to unsafe code

[jamesmunns]

The following is a number of tweaks to the kernel code that in my opinion, make some of the unsafe code a bit more direct to follow. In particular, these changes:

1. Consolidates switch_to as a single method on Task, rather than one version in syscalls.rs, and a handful of semantically-equal copy/pastes
2. Removes as many single/double blind as casts as is reasonable, e.g. x as *const _ as *mut _, replacing them with discrete steps that note the implicit casts (references to pointer), and explicit casts, using ptr::cast()
3. Reduces unsafe usage a bit in startup when initializing each task slot
4. Makes some methods take &mut Task instead of &Task, as these are used for setting CURRENT_TASK_PTR, which semantically allows us to write-through to that address (often in assembly) when returning to the kernel, and that feels a little sketch to me doing from a pointer with shared provenance
5. Adjusts the codegen for Task slots from MaybeUninit<[Task; N]> to [MaybeUninit<Task>; N], as this makes startup a little less awkward

In my opinion, none of these are strictly necessary for soundness, so if we don't want to touch it, I could definitely understand that!

I think 1, 2, and 3 are pretty clear readability/clarity wins.

I think 4 is maybe a little nitpicky, and is a bit more semantically correct, but also unlikely to cause miscompilations in practice.

I think 5 is maybe a mixed bag (we can still make the changes for 3 without 5), and I need to check it isn't going to make humility upset as the types have changed (even though the before/after are guaranteed to have the same layout and semantics).

Happy to revert any chunks, or rework these into separate commits if that makes reviewing easier.

This is extracted from #2592, and most of these were noticed while I was hacking around on that.

[jamesmunns]

Annoyingly, the RAM usage for g070 didn't increase because we are using more, it's just a quirk that the linker isn't packing statics as intelligently as it possibly could.

[aapoalas]

Please excuse my unrequested comments; the topic called out to me.

[jamesmunns]

Hey @labbott or @hawkw, any chance I could get an approve on this? I'd like to get it merged early in this release cycle if possible.

[labbott]

thought I already approved it whoops