simplify the MirPass traits and passes dramatically

Overall goal: reduce the amount of context a mir pass needs so that it
resembles a query.

- The hooks are no longer "threaded down" to the pass, but rather run
  automatically from the top-level (we also thread down the current pass
  number, so that the files are sorted better).
  - The hook now receives a *single* callback, rather than a callback per-MIR.
- The traits are no longer lifetime parameters, which moved to the
  methods -- given that we required
  `for<'tcx>` objecs, there wasn't much point to that.
- Several passes now store a `String` instead of a `&'l str` (again, no
  point).

src/tools/compiletest/src/runtest.rs

@@ -1402,18 +1402,16 @@ actual:\n\
                 }
             }
             MirOpt => {
-                args.extend(["-Z",
-                             "dump-mir=all",
-                             "-Z",
-                             "mir-opt-level=3",
-                             "-Z"]
+                args.extend(["-Zdump-mir=all",
+                             "-Zmir-opt-level=3",
+                             "-Zdump-mir-exclude-pass-number"]
                             .iter()
                             .map(|s| s.to_string()));
 
 
                 let mir_dump_dir = self.get_mir_dump_dir();
                 create_dir_all(mir_dump_dir.as_path()).unwrap();
-                let mut dir_opt = "dump-mir-dir=".to_string();
+                let mut dir_opt = "-Zdump-mir-dir=".to_string();
                 dir_opt.push_str(mir_dump_dir.to_str().unwrap());
                 debug!("dir_opt: {:?}", dir_opt);