Get the default hasher indirectly

`DefaultHasher` wasn't stable until 1.13, at which point all the other
hashers were deprecated, so it's not easy for us to name a hasher type
to use for testing.  However, `RandomState` has been stable since 1.7,
and it implements `BuildHasher` that has a `Hasher` associated type.

bigint/src/lib.rs

@@ -120,8 +120,9 @@ use std::hash;
 
 #[cfg(test)]
 fn hash<T: hash::Hash>(x: &T) -> u64 {
-    use std::hash::Hasher;
-    let mut hasher = std::collections::hash_map::DefaultHasher::new();
+    use std::hash::{BuildHasher, Hasher};
+    use std::collections::hash_map::RandomState;
+    let mut hasher = <RandomState as BuildHasher>::Hasher::new();
     x.hash(&mut hasher);
     hasher.finish()
 }

complex/src/lib.rs

@@ -765,8 +765,9 @@ impl<T> serde::Deserialize for Complex<T> where
 
 #[cfg(test)]
 fn hash<T: hash::Hash>(x: &T) -> u64 {
-    use std::hash::Hasher;
-    let mut hasher = std::collections::hash_map::DefaultHasher::new();
+    use std::hash::{BuildHasher, Hasher};
+    use std::collections::hash_map::RandomState;
+    let mut hasher = <RandomState as BuildHasher>::Hasher::new();
     x.hash(&mut hasher);
     hasher.finish()
 }

rational/src/lib.rs

@@ -670,8 +670,9 @@ impl RatioErrorKind {
 
 #[cfg(test)]
 fn hash<T: hash::Hash>(x: &T) -> u64 {
-    use std::hash::Hasher;
-    let mut hasher = std::collections::hash_map::DefaultHasher::new();
+    use std::hash::{BuildHasher, Hasher};
+    use std::collections::hash_map::RandomState;
+    let mut hasher = <RandomState as BuildHasher>::Hasher::new();
     x.hash(&mut hasher);
     hasher.finish()
 }