rename send_map to hashmap

This makes the module much more discoverable, and is in line with the
'treemap' naming.

src/test/bench/core-map.rs

@@ -17,7 +17,7 @@
 extern mod std;
 use std::map;
 use core::mutable::Mut;
-use core::send_map::linear::*;
+use core::hashmap::linear::*;
 use core::io::WriterUtil;
 
 struct Results {

src/test/run-pass/issue-4016.rs

@@ -11,7 +11,7 @@
 // xfail-test
 extern mod std;
 
-use send_map::linear;
+use hashmap::linear;
 use std::json;
 use std::serialization::{Deserializable, deserialize};