stdlib: Stop incurring vtable dispatch costs when hashmaps are used

This required changing almost all users of hashmaps to import the hashmap interface first.

The `size` member in the hashmap structure was renamed to `count` to work around a name conflict.

src/libstd/json.rs

@@ -5,6 +5,7 @@ import result::{ok, err};
 import io;
 import io::{reader_util, writer_util};
 import map;
+import map::hashmap;
 
 export json;
 export error;
@@ -36,7 +37,7 @@ enum json {
     /* Variant: list */
     list([json]),
     /* Variant: dict */
-    dict(map::map<str,json>),
+    dict(map::hashmap<str,json>),
     /* Variant: null */
     null,
 }