syntax: Eliminate 'mutable' keyword. Closes #2254

src/test/compile-fail/ctor-uninit-var.rs

@@ -1,7 +1,7 @@
 // error-pattern:unsatisfied precondition
 class cat {
   priv {
-    let mutable meows : uint;
+    let mut meows : uint;
   }
 
   let how_hungry : int;

src/test/compile-fail/mutable-class-fields-2.rs

@@ -1,7 +1,7 @@
 // error-pattern:assigning to immutable field
 class cat {
   priv {
-    let mutable meows : uint;
+    let mut meows : uint;
   }
 
   let how_hungry : int;

src/test/compile-fail/mutable-class-fields.rs

@@ -1,7 +1,7 @@
 // error-pattern:assigning to immutable field
 class cat {
   priv {
-    let mutable meows : uint;
+    let mut meows : uint;
   }
 
   let how_hungry : int;