Change decl to local in expr_for and expr_for_each

Since the decl in a for or for-each loop must always be a local
decl, I changed the AST to express this. Fewer potential match
failures and "the impossible happened" error messages = yay!

src/comp/util/common.rs

@@ -224,7 +224,7 @@ fn has_nonlocal_exits(&ast::block b) -> bool {
     ret *has_exits;
 }
 
-fn local_rhs_span(&@ast::local l, &span def) -> span {
+fn local_rhs_span(&@ast::local_ l, &span def) -> span {
     alt (l.init) {
         case (some(?i)) { ret i.expr.span; }
         case (_) { ret def; }