Also, fix rustdoc warnings.

mutability

as `Applicability::Unspecified`

Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>

Suggest deref when coercing `ty::Ref` to `ty::RawPtr` with arbitrary mutability

Fixes rust-lang#71676
1. Implement dereference suggestion when coercing `ty::Ref` to `ty::RawPtr` with arbitrary mutability.
2. Extract the dereference steps into `deref_steps()`, which removes all the `use` and `pub` noise introduced by last PR rust-lang#71540, and makes the code more readable.
3. Use the `remove_prefix()` closure which makes the prefix removal more readable.
4. Introduce `Applicability` as a return value of `check_ref` to suggest `Applicability::Unspecified` suggestion.

**Special**: I found it is not possible to genereate `Applicability::MachineApplicable` suggestion for situation like this:
```rust
use std::ops::Deref;
use std::ops::DerefMut;
struct Bar(u8);
struct Foo(Bar);
struct Emm(Foo);
impl Deref for Bar{
    type Target = u8;
    fn deref(&self) -> &Self::Target {
        &self.0
    }
}
impl Deref for Foo {
    type Target = Bar;
    fn deref(&self) -> &Self::Target {
        &self.0
    }
}
impl Deref for Emm {
    type Target = Foo;
    fn deref(&self) -> &Self::Target {
        &self.0
    }
}
impl DerefMut for Bar{
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.0
    }
}
impl DerefMut for Foo {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.0
    }
}
impl DerefMut for Emm {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.0
    }
}
fn main() {
    let a = Emm(Foo(Bar(0)));
    let _: *mut u8 = &a; //~ ERROR mismatched types
}
```
We may suggest `&mut ***a` here, but the `a` is not declared as mutable variable. And also when processing HIR, it's not possible to check if `a` is declared as a mutable variable (currently we do borrow checking with MIR). So we cannot ensure that suggestion when coercing immutable reference to mutable pointer is always machine applicable. Therefore I added a `Applicability` return value in `check_ref()`. And move the `immutable reference -> mutable pointer` situation into a sperate test file without `run-rustfix`. (It seems that `run-rustfix` will also adopt `Applicability::Unspecified` suggestion, which is strange)

…vink

doc: make Stack and StackElement a little pretty

Also, fix rustdoc warnings.

cleanup: `config::CrateType` -> `CrateType`

Remove recommendation for unmaintained dirs crate

See rust-lang#71684 for reasoning here

fix rustdoc warnings