rewrite 'mutable &' as '& mutable', corresponding to grammar shift.

src/test/run-pass/lazy-and-or.rs

@@ -1,4 +1,4 @@
-fn incr(mutable &int x) -> bool {
+fn incr(& mutable int x) -> bool {
   x += 1;
   check (false);
   ret false;

src/test/run-pass/writealias.rs

@@ -2,7 +2,7 @@
 
 type point = rec(int x, int y, mutable int z);
 
-fn f(mutable &point p) {
+fn f(& mutable point p) {
   p.z = 13;
 }