Use attributes for native module ABI and link name

This patch changes how to specify ABI and link name of a native module.

Before:
  native "cdecl" mod llvm = "rustllvm" {...}

After:
  #[abi = "cdecl"]
  #[link_name = "rustllvm"]
  native mod llvm {...}

The old optional syntax for ABI and link name is no longer supported.

Fixes issue #547

src/comp/front/config.rs

@@ -45,8 +45,7 @@ fn fold_native_mod(cfg: ast::crate_cfg, nm: ast::native_mod,
                    fld: fold::ast_fold) -> ast::native_mod {
     let filter = bind filter_native_item(cfg, _);
     let filtered_items = vec::filter_map(filter, nm.items);
-    ret {native_name: nm.native_name,
-         abi: nm.abi,
+    ret {abi: nm.abi,
          view_items: vec::map(fld.fold_view_item, nm.view_items),
          items: filtered_items};
 }