Move mutability checking into its own pass.

Having it in the alias pass was slightly more efficient (finding
expression roots has to be done in both passes), but further muddled
up the already complex alias checker.

Also factors out some duplication in the mutability-checking code.

Author: marijnh

src/comp/driver/rustc.rs

@@ -165,8 +165,10 @@ fn compile_input(sess: session::session, cfg: ast::crate_cfg, input: &istr,
         time(time_passes, ~"typestate checking",
              bind middle::tstate::ck::check_crate(ty_cx, crate));
     }
-    let mut_map = time(time_passes, ~"alias checking",
-                       bind middle::alias::check_crate(ty_cx, crate));
+    let mut_map = time(time_passes, ~"mutability checking",
+                       bind middle::mut::check_crate(ty_cx, crate));
+    time(time_passes, ~"alias checking",
+         bind middle::alias::check_crate(ty_cx, crate));
     time(time_passes, ~"kind checking",
          bind kind::check_crate(ty_cx, crate));
     if sess.get_opts().no_trans { ret; }