std: Fill in all missing imports

Fallout from the previous commits

src/libstd/unstable/mutex.rs

@@ -167,6 +167,7 @@ mod imp {
     use libc::c_void;
     use libc;
     use ptr;
+    use ptr::RawPtr;
 
     type pthread_mutex_t = libc::c_void;
     type pthread_mutexattr_t = libc::c_void;

src/libstd/unstable/stack.rs

@@ -24,6 +24,8 @@
 //! detection is not guaranteed to continue in the future. Usage of this module
 //! is discouraged unless absolutely necessary.
 
+use str::Str;
+
 static RED_ZONE: uint = 20 * 1024;
 
 /// This function is invoked from rust's current __morestack function. Segmented