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/librustc_driver/driver.rs

@@ -979,8 +979,7 @@ pub fn phase_3_run_analysis_passes<'tcx, F, R>(sess: &'tcx Session,
             passes.push_pass(box mir::transform::simplify::SimplifyCfg::new("initial"));
             passes.push_pass(box mir::transform::type_check::TypeckMir);
             passes.push_pass(box mir::transform::qualify_consts::QualifyAndPromoteConstants);
-            passes.push_pass(
-                box mir::transform::simplify_branches::SimplifyBranches::new("initial"));
+            passes.push_pass(box mir::transform::simplify_branches::SimplifyBranches::new("initial"));
             passes.push_pass(box mir::transform::simplify::SimplifyCfg::new("qualify-consts"));
             // And run everything.
             passes.run_passes(tcx);