Add #[serde(rename_all_fields = "foo")] attribute

[jplatte]

fixes #1061

If you don't like the style change in the second commit, I can remove it from this PR (or you can just cherry-pick the first commit).

[jplatte]

r? @dtolnay

[jplatte]

I'll try again since Triagebot Notifications are now a thing:

r? @dtolnay

I've opened this PR because you wrote over at https://github.com/serde-rs/serde/issues/1061#issuecomment-451716365 that you'd accept a PR for this. If your opinion has changed, I won't be upset if this is closed without further reasoning.

[kjeremy]

@dtolnay it looks like this was approved a few months back but not merged in.

[jplatte]

@kjeremy The person who left the review doesn't seem to be associated with serde in any way.

[jplatte]

Rebased to trigger new CI runs, the old ones had been stuck for months.

[jplatte]

CI fixed, no merge conflics. Ping @dtolnay

[roberts-pumpurs]

Are there no plans on merging this in? This really would be a feature I'd love to have.

[kaiba42]

+1 on this merge...

[llgerard]

Would love this!

[pmhpereira]

Any update on this? Would love to have this merged as well.

[jplatte]

Rebased to fix conflicts with the syn 2.0 upgrade.