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/json.rs

@@ -1208,6 +1208,7 @@ mod tests {
     use core::prelude::*;
 
     use json::*;
+    use serialize;
 
     use core::result;
     use core::hashmap::linear::LinearMap;