Auto merge of #129845 - scottmcm:redo-layout, r=Noratrieb

Take more advantage of the `isize::MAX` limit in `Layout`

Things like `padding_needed_for` are current implemented being super careful to handle things like `Layout::size` potentially being `usize::MAX`.

But now that #95295 has happened, that's no longer a concern.  It's possible to add two `Layout::size`s together without risking overflow now.

So take advantage of that to remove a bunch of checked math that's not actually needed.  For example, the round-up-and-add-next-size in `extend` doesn't need any overflow checks at all, just the final check for compatibility with the alignment.

(And while I was doing that I made it all unstably const, because there's nothing in `Layout` that's fundamentally runtime-only.)