Add an overloaded [] function to the map interface. Closes #2730.

src/libstd/map.rs

@@ -45,6 +45,9 @@ iface map<K, V: copy> {
     "]
     fn get(K) -> V;
 
+    #[doc = "Like get, but as an operator."]
+    fn [](K) -> V;
+
     #[doc = "
     Get the value for the specified key. If the key does not exist in
     the map then returns none.
@@ -232,6 +235,10 @@ mod chained {
             option::get(self.find(k))
         }
 
+        fn [](k: K) -> V {
+            option::get(self.find(k))
+        }
+
         fn remove(k: K) -> option<V> {
             alt self.search_tbl(k, self.hasher(k)) {
               not_found {none}

src/libstd/smallintmap.rs

@@ -79,6 +79,7 @@ impl <V: copy> of map::map<uint, V> for smallintmap<V> {
         contains_key(self, key)
     }
     fn get(&&key: uint) -> V { get(self, key) }
+    fn [](&&key: uint) -> V { get(self, key) }
     fn find(&&key: uint) -> option<V> { find(self, key) }
     fn rehash() { fail }
     fn each(it: fn(&&uint, V) -> bool) {