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

[seijikun]

Hm. The Lint failures seem to be outside the code I changed in this PR

[nicholasbishop]

Probably due to 1.86 going stable recently. If you want to, please put up a separate pr with fixes for those lints. Or I can do it sometime this weekend probably.

[phip1611]

I've set up #1602.

Thanks for your patience, @seijikun. We are almost there!

[phip1611]

LGTM now!