Separate `Analysis` and `Results`.

`Results` contains and `Analysis` and an `EntryStates`. The unfortunate
thing about this is that the analysis needs to be mutable everywhere
(`&mut Analysis`) which forces the `Results` to be mutable everywhere,
even though `EntryStates` is immutable everywhere.

To fix this, this commit renames `Results` as `AnalysisAndResults`,
renames `EntryStates` as `Results`, and separates the analysis and
results as much as possible. (`AnalysisAndResults` doesn't get much use,
it's mostly there to facilitate method chaining of
`iterate_to_fixpoint`.)

`Results` is immutable everywhere, which:
- is a bit clearer on how the data is used,
- avoids an unnecessary clone of entry states in
  `locals_live_across_suspend_points`, and
- moves the results outside the `RefCell` in Formatter.

The commit also reformulates `ResultsHandle` as the generic `CowMut`,
which is simpler than `ResultsHandle` because it doesn't need the
`'tcx` lifetime and the trait bounds. It also which sits nicely
alongside the new use of `Cow` in `ResultsCursor`.

src/mir/dataflow.md

@@ -148,8 +148,7 @@ whereas this code uses [`ResultsCursor`]:
 
 ```rust,ignore
 let mut results = MyAnalysis::new()
-    .into_engine(tcx, body, def_id)
-    .iterate_to_fixpoint()
+    .iterate_to_fixpoint(tcx, body, None);
     .into_results_cursor(body);
 
 // Inspect the fixpoint state immediately before each `Drop` terminator.