Add preview feature

Author: AdamNiederer

src/lib.rs

@@ -198,6 +198,7 @@
 #![cfg_attr(feature = "no-std", no_std)]
 #![cfg_attr(test, feature(test))]
 #![cfg_attr(test, feature(inclusive_range))]
+#![feature(rust_2018_preview)]
 
 mod core_or_std {
     #[cfg(not(feature = "no-std"))] pub use ::std::*;