Decide whether to version codegen/macros separately from serde

[dtolnay]

We can keep them in the same repository, but do we need to continue tying their version numbers together? It often happens that we change codegen/macros and cut a release in which serde hasn't changed at all, or vice versa.

[dtolnay]

We can reopen if anyone has a strong opinion. For now I think serde and serde_codegen make breaking changes infrequently enough that tying the versions is not a problem.

[dtolnay]

Reopening to discuss the possibility of releasing serde_codegen 0.9 while staying with serde 0.8. I need to fix the cargo version issue (#543) in order to make progress on serde_derive (#548). This would be a breaking change for serde_codegen but there is no real motivation for breaking core serde right now - the enum API I guess (#523) but that can wait.

Another option is to go all-in on Macros 1.1. Basically take all the code currently in serde_codegen and fold it into serde_derive, then delete serde_codegen and serde_macros from the codebase and never release another version of them. In the event that someone demands a codegen bug fixed on stable, we can fix it on an old 0.8 branch.

@erickt @oli-obk

[dtolnay]

Never mind, I think I can work around it for now in serde_codegen as long as I fix it in syntex/quasi/aster, which is easier.