Replace layout computations with `alloc::Layout`?

[joshlf]

Now that some alloc::Layout methods are const, we may be able to replace some of our layout module machinery with these methods. This will only work on recent toolchains, so we will still need to fall back to our implementations on older toolchains, but it will be good to rely on more widely-used implementations where possible, and we will eventually be able to remove our implementations when we bump our MSRV.