Add 'CamelCase' to doc_valid_idents

Fixes #2395

…eferences, r=RalfJung

Make file descriptors into refcount references

fixes rust-lang#3525

Remove `fn dup` in `trait FileDescription`, define `struct FileDescriptor(Rc<RefCell<dyn FileDescription>>)`, and use `BTreeMap<i32, FileDescriptor>` in `FdTable`.

---

There are some refactors similar to the following form:
```rust
{  // origin:
    if let Some(file_descriptor) = this.machine.fds.get_mut(fd) {
        // write file_descriptor
        this.try_unwrap_io_result(result)
    } else {
        this.fd_not_found()
    }
}
{  // now:
    let Some(mut file_descriptor) = this.machine.fds.get_mut(fd) else {
        return this.fd_not_found();
    };
    // write file_descriptor
    drop(file_descriptor);
    this.try_unwrap_io_result(result)
}
```
The origin form can't compile because as using `RefCell` to get interior mutability, `fn get_mut` return `Option<std::cell::RefMut<'_, dyn FileDescription>>` instead of `Option<&mut dyn FileDescription>` now, and the `deref_mut` on `file_descriptor: RefMut` will cause borrow `this` as mutable more than once at a time.
So this form of refactors and manual drops are are implemented to avoid borrowing `this` at the same time.