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/rustdoc/tystr_pass.rs

@@ -4,6 +4,7 @@
 import rustc::syntax::ast;
 import rustc::syntax::print::pprust;
 import rustc::middle::ast_map;
+import std::map::hashmap;
 
 export mk_pass;