Rename rayon_unstable flag

[dtolnay]

In proc-macro2 we love how rayon_unstable has worked for Rayon and we have adopted a similar RUSTFLAGS-based unstable setting. Discussion where we decided to go this route begins at dtolnay/proc-macro2#36 (comment). We originally called the flag procmacro2_unstable to mimic Rayon.

The problem is we already have a Cargo cfg that enables some things that require a nightly compiler, and this was called --features proc-macro2/unstable. These are basically orthogonal things -- APIs that are still being designed and are exempt from semver but do not require a nightly compiler, and APIs that are semver guaranteed but require nightly features. It was confusing to call both of these things "unstable". Discussion: dtolnay/proc-macro2#42, PR: dtolnay/proc-macro2#44.

We settled on --cfg procmacro2_semver_exempt which is more suggestive of the way in which those things are unstable. Just opening this issue to suggest renaming Rayon's flag to rayon_semver_exempt if you think that would be valuable.

@alexcrichton @mystor

[nikomatsakis]

I don't have a strong opinion about this -- semver_exempt seems kinda' wordy, but I agree that overloading unstable is perhaps unwise.

[cuviper]

We removed that in #716, but if we ever reintroduce it, we could consider a new name.