make mut a keyword synonymous with mutable

first step towards issue #1273

src/comp/syntax/ast_util.rs

@@ -66,8 +66,8 @@ pure fn lazy_binop(b: binop) -> bool {
 
 fn unop_to_str(op: unop) -> str {
     alt op {
-      box(mt) { if mt == mut { ret "@mutable "; } ret "@"; }
-      uniq(mt) { if mt == mut { ret "~mutable "; } ret "~"; }
+      box(mt) { if mt == m_mutbl { ret "@mut "; } ret "@"; }
+      uniq(mt) { if mt == m_mutbl { ret "~mut "; } ret "~"; }
       deref { ret "*"; }
       not { ret "!"; }
       neg { ret "-"; }