Consider switching to autocfg from rustc_version

[matklad]

It's very likely that https://docs.rs/autocfg/1.0.0/autocfg/ becomes the crate for rustc feature detection:

• it's 1.0
• it's maintained by a high-profile Rust contributor
• it is based on a generally better patrern of feature detection (as opposed to version sniffing)

It might make sense to replace rustc_version with autocfg

[Gilnaa]

Great idea, I'll try to do it soon