strengthen Idx to require Ord + Hash

You should always be able to know that any `T` where `T: Idx`
can be used in a `BTreeMap` and a `FxHashMap`.

src/librustc_data_structures/indexed_vec.rs

@@ -14,6 +14,7 @@ use std::slice;
 use std::marker::PhantomData;
 use std::ops::{Index, IndexMut, Range, RangeBounds};
 use std::fmt;
+use std::hash::Hash;
 use std::vec;
 use std::u32;
 
@@ -22,7 +23,7 @@ use rustc_serialize as serialize;
 /// Represents some newtyped `usize` wrapper.
 ///
 /// (purpose: avoid mixing indexes for different bitvector domains.)
-pub trait Idx: Copy + 'static + Eq + Debug {
+pub trait Idx: Copy + 'static + Ord + Debug + Hash {
     fn new(idx: usize) -> Self;
     fn index(self) -> usize;
 }