Rollup merge of #95361 - scottmcm:valid-align, r=Mark-Simulacrum

Rollup merge of #95361 - scottmcm:valid-align, r=Mark-Simulacrum

Make non-power-of-two alignments a validity error in `Layout`

Inspired by the zulip conversation about how `Layout` should better enforce `size <= isize::MAX as usize`, this uses an N-variant enum on N-bit platforms to require at the validity level that the existing invariant of "must be a power of two" is upheld.

This was MIRI can catch it, and means there's a more-specific type for `Layout` to store than just `NonZeroUsize`.

It's left as `pub(crate)` here; a future PR could consider giving it a tracking issue for non-internal usage.