Built-in derive macros `Encodable`, `Decodable`, `RustcEncodable` and `RustcDecodable` are stable

[petrochenkov]

The macros produce paths like serialize::foo or rustc_serialize::foo and I suspect that the feature gating was supposed to happen when you write extern crate serialize to bring that serialize into scope.

However, you can easily write use anything as serialize, and rustc_serialize is a stable crate on crates.io, so this kind of feature gating doesn't work.

After #62042 lands we should try to un-stabilize them and test it with crater.

[petrochenkov]

The bench attribute macro is also stable.

[pnkfelix]

pre-triage: P-medium. removing nomination.

@petrochenkov : do you want to be assigned to this? Or are you just sending out a general alert, but do not plan to fix it?

[petrochenkov]

@pnkfelix
I can make a PR for crater once #62042 lands.