The current order is a mess.

`lib.rs` is a strange place for it, and it's only used within
`rustc_peek.rs`, so it doesn't need to be `pub`.

It's not useful, and only obfuscates things.

`Forward` is the default.

All the fields are named.

Some types have a `body: &'mir Body<'tcx>` and some have `body: &'a
Body<'tcx>`. The former is more readable, so this commit converts some
fo the latter to the former.

`body` is already a `&Body`.

`on_all_children_bits` has two arguments that are unused: `tcx` and
`body`. This was not detected by the compiler because it's a recursive
function.

This commit removes them, and removes lots of other arguments and fields
that are no longer necessary.

`mut_results` immediately below is the `&mut self` version, this one
should be `&self`.

We can just call `ResultsCursor::state` and `ResultsCursor::analysis`
separately.