mode: reset mode on entry to fn body.

This is an interim fix to address the "Beware!" unsoundness.  I have a
more comprehensive rewrite of mode.rs in the pipeline.

r=pcwalton

src/test/run-fail/unwind-lambda.rs

@@ -15,7 +15,7 @@ fn main() {
     let carrots = @~"crunchy";
 
     fn@(tasties: @~str, macerate: fn(~str)) {
-        macerate(*tasties);
+        macerate(copy *tasties);
     } (carrots, |food| {
         let mush = food + cheese;
         let f = fn@() {