Replace Fn impls with RPIT impls in rustc_index

This is cleaner and removes an unstable feature usage

compiler/rustc_index/src/lib.rs

@@ -4,7 +4,6 @@
 #![feature(iter_zip)]
 #![feature(min_specialization)]
 #![feature(test)]
-#![feature(fn_traits)]
 
 pub mod bit_set;
 pub mod vec;