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/compile-fail/access-mode-in-closures.rs

@@ -0,0 +1,10 @@
+enum sty = ~[int];
+
+fn unpack(unpack: &fn(v: &sty) -> ~[int]) {}
+
+fn main() {
+    let foo = unpack(|s| {
+        // Test that `s` is moved here.
+        match *s { sty(v) => v } //~ ERROR moving out of dereference of immutable & pointer
+    });
+}

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@() {

src/test/run-pass/last-use-corner-cases.rs

@@ -39,6 +39,6 @@ fn main() {
     // Verify that blocks can't interfere with each other.
     fn two_blocks(a: fn(), b: fn()) { a(); b(); a(); b(); }
     let q = ~50;
-    two_blocks(|| { let a = q; assert *a == 50;},
-               || { let a = q; assert *a == 50;});
+    two_blocks(|| { let a = copy q; assert *a == 50;},
+               || { let a = copy q; assert *a == 50;});
 }

src/test/run-pass/monad.rs

@@ -43,6 +43,8 @@ fn transform(x: Option<int>) -> Option<~str> {
 fn main() {
     assert transform(Some(10)) == Some(~"11");
     assert transform(None) == None;
-    assert (~[~"hi"]).bind(|x| ~[x, x + ~"!"] ).bind(|x| ~[x, x + ~"?"] ) ==
+    assert (~[~"hi"])
+        .bind(|x| ~[copy x, x + ~"!"] )
+        .bind(|x| ~[copy x, x + ~"?"] ) ==
         ~[~"hi", ~"hi?", ~"hi!", ~"hi!?"];
 }