helpers: Add AlignedBuffer

[seijikun]

AlignedBuffer is a helper class that manages the livetime of a memory region, allocated using a certain alignment. Like Box, it handles deallocation when the object isn't used anymore.

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[nicholasbishop]

As mentioned in #1595 (comment), let's move to the mem module.

[phip1611]

Thanks for your many contributions and your patience! I left a few remarks.

If you resolve them, we are good to go

[phip1611]

I'm not sure if it is nice to just panic instead of report an error. Perhaps we need a

enum AlignedBufferError {
    Layout(LayoutError),
    OutOfMemory,
}

error type that is also returned by the constructors.

I'd like to hear @nicholasbishop opinion on that

[nicholasbishop]

I don't have a very strong opinion either way. Rust does tend to just panic on allocation failures in std, although they later added things like https://doc.rust-lang.org/std/vec/struct.Vec.html#method.try_reserve to avoid panic. I think allocation failing is theoretically more likely in UEFI than in a hosted environment, since there's (presumably) no overcommit in a UEFI environment. However, in practice an allocation failure is very unlikely so perhaps it's fine to just panic.

With that background, my opinion is: let's keep these functions panicking, and we can easily add non-panicking constructors in the future without breaking compatibility.

Please add a # Panics section in the docstrings of these two constructors so that it's clear for callers what the behavior is.