[WIP: do not merge] fixup/ci: feature-gate stabilized compiler features

[rmsyn]

Adds conditional feature-gates based on rustc version to only include unstable features on older compiler versions.

Introduces the rustversion dependency to handle the feature-gates.

Fixes breaking CI builds.

[rmsyn]

This fix is broken due to proc_macro errors, so another solution for conditional compilation based on rustc version is needed, or some fix for the proc_macro errors.

... or you could just bump the MSRV ...