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/libsyntax/fold.rs

@@ -12,7 +12,8 @@ use core::prelude::*;
 
 use ast::*;
 use ast;
-use codemap::span;
+use codemap::{span, spanned};
+use opt_vec::OptVec;
 
 use core::option;
 use core::vec;