Simplify macros and prepare 0.38.0 release

[SimonSapin]

Release motivated by #433

Drive-by cleanup: with MSRV past inline consts we no longer need the MaybeUninit::<[MaybeUninit<u8>; N]>::uninit().assume_init() trick.

[Loirooriol]

Any reason 0.38 wasn't published to crates.io? Stylo needs eff8704 to import https://phabricator.services.mozilla.com/D317537

[SimonSapin]

No reason to delay it if we need it, I just haven’t needed it yet for servo/servo#47431. I few other PRs have been merged since this one, so pick whichever point in the history is more convenient for the Gecko import. Just make sure to push a corresponding git tag (or I can help do that, ping me wherever)

[Loirooriol]

I think it would be confusing to publish 0.38 as a later commit. So I think 0.38 should be published here on 30658ed.

And then Stylo will actually need a 0.39 that includes 84af796

[SimonSapin]

I think that’s what git tags are for but if you feel strongly about it go ahead. (Do you have access?)