HashCache: Apply trait bounds on type itself, rather than only the impl

I don't think there is any reason to keep the container more generic
than its (only) implementation -- the constraints are necessary for this
container to work at all.

Defining the constraints on the type itself also enables use
of #[derive(Debug)].

components/util/cache.rs

@@ -12,7 +12,10 @@ use std::hash::{Hash, Hasher, SipHasher};
 use std::slice::Iter;
 
 
-pub struct HashCache<K, V> {
+pub struct HashCache<K, V>
+    where K: Clone + PartialEq + Eq + Hash,
+          V: Clone,
+{
     entries: HashMap<K, V, DefaultState<SipHasher>>,
 }