Rollup merge of #51226 - gnzlbg:nonzero_align, r=SimonSapin

Make Layout's align a NonZeroUsize

This PR makes the `Layout`'s align field a `NonZeroUsize` since it cannot ever be zero, not even while building a `Layout`. It also contains some drive-by minor cleanups over the docs and the code, like updating the documented error types, or using the `size()` and `align()` methods instead of accessing the fields directly (the latter was required for the `NonZeroUsize` change anyways).

r? @SimonSapin

cc @Amanieu

diff --cc src/libcore/alloc.rs
Simple merge