libcore: minor code cleanup.

This is minor and probably completely inconsequential to performance,
but I find vec::map to be more clear than vec::each and a push.

src/libsyntax/attr.rs

@@ -90,9 +90,7 @@ fn attr_meta(attr: ast::attribute) -> @ast::meta_item { @attr.node.value }
 
 // Get the meta_items from inside a vector of attributes
 fn attr_metas(attrs: ~[ast::attribute]) -> ~[@ast::meta_item] {
-    let mut mitems = ~[];
-    for attrs.each |a| { mitems.push(attr_meta(*a)); }
-    return mitems;
+    do attrs.map |a| { attr_meta(*a) }
 }
 
 fn desugar_doc_attr(attr: &ast::attribute) -> ast::attribute {