Stop unloading syntax libraries

Externally loaded libraries are able to do things that cause references
to them to survive past the expansion phase (e.g. creating @-box cycles,
launching a task or storing something in task local data). As such, the
library has to stay loaded for the lifetime of the process.

src/libsyntax/ext/base.rs

@@ -20,7 +20,6 @@ use parse::token::{InternedString, intern, str_to_ident};
 use util::small_vector::SmallVector;
 
 use std::hashmap::HashMap;
-use std::unstable::dynamic_lib::DynamicLibrary;
 
 // new-style macro! tt code:
 //
@@ -143,8 +142,6 @@ pub struct BlockInfo {
     macros_escape : bool,
     // what are the pending renames?
     pending_renames : RenameList,
-    // references for crates loaded in this scope
-    macro_crates: ~[DynamicLibrary],
 }
 
 impl BlockInfo {
@@ -152,7 +149,6 @@ impl BlockInfo {
         BlockInfo {
             macros_escape: false,
             pending_renames: ~[],
-            macro_crates: ~[],
         }
     }
 }
@@ -551,10 +547,6 @@ impl SyntaxEnv {
         self.find_escape_frame().map.insert(k, v);
     }
 
-    pub fn insert_macro_crate(&mut self, lib: DynamicLibrary) {
-        self.find_escape_frame().info.macro_crates.push(lib);
-    }
-
     pub fn info<'a>(&'a mut self) -> &'a mut BlockInfo {
         &mut self.chain[self.chain.len()-1].info
     }