Update allocator-api2

[zakarumych]

New version fixes some inconsistencies with std

[zakarumych]

Build error in bumpalo encountered by CI is fixed in fitzgen/bumpalo#299

[cuviper]

Just to be explicit, this is definitely a breaking change -- as the bumpalo failure demonstrates.

[zakarumych]

Bumpalo fails without this change as well.

[zakarumych]

But it is a breaking change for users who use allocator-api2 for hashbrown types, as they would need to upgrade their version.

The minor version update is forced by new implementation Allocator for &mut A, that was added to core.

[cuviper]

But it is a breaking change for users who use allocator-api2 for hashbrown types, as they would need to upgrade their version.

That's really what I meant, as it's (implicitly) a public dependency. And this is fine for hashbrown to change IMO, as long as we do bump semver for it.

I admit I didn't look at what actually went wrong with bumpalo in particular.