`Formatter` currently has a `RefCell<Option<Results>>` field. This is so
the `Results` can be temporarily taken and put into a `ResultsCursor`
that is used by `BlockFormatter`, and then put back, which is messy.
This commit changes `Formatter` to have a `RefCell<ResultsCursor>` and
`BlockFormatter` to have a `&mut ResultsCursor`, which greatly
simplifies the code at the `Formatter`/`BlockFormatter` interaction
point in `Formatter::node_label`. It also means we construct a
`ResultsCursor` once per `Formatter`, instead of once per `node_label`
call.
The commit also:
- documents the reason for the `RefCell`;
- adds a `Formatter::body` method, replacing the `Formatter::body`
field.
Convert all the crates that have had their diagnostic migration
completed (except save_analysis because that will be deleted soon and
apfloat because of the licensing problem).
Make it explicit that the analysis direction is constant.
This also makes the value immediately available for optimizations.
Previously those functions were neither inline nor generic and so their
definition was unavailable when using data flow framework from other
crates.
