Have a way to write different code based on the runtime detected features

[parched]

It would be good if the user could write different code based on which version runtime features were available. For example, one might want to write inline assembly when some vector instructions are available.

IMO, this would best be done using #[cfg(target_feature)] inside the functions but that is currently blocked on rust-lang/rust#39336 and rust-lang/rust#42515.