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/rustc/metadata/decoder.rs

@@ -1,6 +1,7 @@
 // Decoding metadata from a single crate's metadata
 
 import std::{ebml, map, io};
+import std::map::hashmap;
 import io::writer_util;
 import syntax::{ast, ast_util};
 import driver::session::session;