Overhaul `syntax::fold::Folder`.

This commit changes `syntax::fold::Folder` from a functional style
(where most methods take a `T` and produce a new `T`) to a more
imperative style (where most methods take and modify a `&mut T`), and
renames it `syntax::mut_visit::MutVisitor`.

The first benefit is speed. The functional style does not require any
reallocations, due to the use of `P::map` and
`MoveMap::move_{,flat_}map`. However, every field in the AST must be
overwritten; even those fields that are unchanged are overwritten with
the same value. This causes a lot of unnecessary memory writes. The
imperative style reduces instruction counts by 1--3% across a wide range
of workloads, particularly incremental workloads.

The second benefit is conciseness; the imperative style is usually more
concise. E.g. compare the old functional style:
```
fn fold_abc(&mut self, abc: ABC) {
    ABC {
        a: fold_a(abc.a),
        b: fold_b(abc.b),
        c: abc.c,
    }
}
```
with the imperative style:
```
fn visit_abc(&mut self, ABC { a, b, c: _ }: &mut ABC) {
    visit_a(a);
    visit_b(b);
}
```
(The reductions get larger in more complex examples.)

Overall, the patch removes over 200 lines of code -- even though the new
code has more comments -- and a lot of the remaining lines have fewer
characters.

Some notes:

- The old style used methods called `fold_*`. The new style mostly uses
  methods called `visit_*`, but there are a few methods that map a `T`
  to something other than a `T`, which are called `flat_map_*` (`T` maps
  to multiple `T`s) or `filter_map_*` (`T` maps to 0 or 1 `T`s).

- `move_map.rs`/`MoveMap`/`move_map`/`move_flat_map` are renamed
  `map_in_place.rs`/`MapInPlace`/`map_in_place`/`flat_map_in_place` to
  reflect their slightly changed signatures.

- Although this commit renames the `fold` module as `mut_visit`, it
  keeps it in the `fold.rs` file, so as not to confuse git. The next
  commit will rename the file.

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