Tracking: More principled alignment tracking, enforcement throughout kernel

[ppannuto]

We've run into unexpected errors with alignment a few times, mostly because of hidden expectations around alignment that JustWorked when we were getting things working with simpler code and bigger buffers, but adding more architectures (esp. host-based unit testing), more complex process management, etc have revealed some problems waiting to happen.

I think we ultimately need to have some kind of AlignedBuffer type, or other interface constraint that low-level subsystems (especially arch-specific stuff like context switching machinery) can impose on their (arch-independent) callers when alignment assumptions / requirements exist.

I suspect this is something that can/should be zero-overhead, an associated zero-sized alignment type or other Rust-y game, but there is a bit of a design exercise that we need to go through for this.