Fix implicit leaks of imports throughout libraries

Also touch up use of 'pub' and move some tests around so the tested functions
don't have to be 'pub'

src/libstd/arc.rs

@@ -497,6 +497,7 @@ mod tests {
     use arc::*;
     use arc;
 
+    use core::cell::Cell;
     use core::option::{Some, None};
     use core::option;
     use core::pipes;