Memory page size not aligned to password length

[seqre]

As described in cot-rs/cot#328 (comment) by @Kijewski:

If the password length is less than page sized (which is somewhat ensured, as macOS uses 16k pages if I'm not mistaken), and two passwords get allocated in the same page, then dropping one password will munlock the other password. Fixing this problem would be difficult, I think. Either allocate pages manually for every password or have some sort of arena that stores all passwords. I hate the latter option, though, because that arena would be treasure chest for any evil-doer .