Assess the impact of rust-lang/rust#95295

[yvt]

rust-lang/rust#95295 has made it a library UB to pass a size that does not fit in isize toLayout::from_size_align_unchecked.

• We might be able to take advantage of this to remove a few overflow checks. However, it should be kept in mind that Rust versions prior to 1.65.0 are unaffected.
• We should review our own uses of Layout::from_size_align[_unchecked].