fixup mutability of vec::each, make iter_bytes pure

also, change DVec() to work with imm vectors rather than mut ones

src/libsyntax/ast_util.rs

@@ -254,7 +254,7 @@ pure fn is_call_expr(e: @expr) -> bool {
 // This makes def_id hashable
 impl def_id : core::to_bytes::IterBytes {
     #[inline(always)]
-    fn iter_bytes(lsb0: bool, f: core::to_bytes::Cb) {
+    pure fn iter_bytes(lsb0: bool, f: core::to_bytes::Cb) {
         core::to_bytes::iter_bytes_2(&self.crate, &self.node, lsb0, f);
     }
 }