to_gd(), Gd::try_cast() result, formatting

[Bromeon]

You can now do this:

use godot::prelude::*;

#[derive(GodotClass)]
struct MyClass {
    #[base]
    my_base: Base<RefCounted>,
}

impl MyClass {
    fn some_method(&self) {
        let outer_ptr: Gd<Self> = self.self_gd();
    }
}

A #[base] field must be present, otherwise self_gd (and its extension trait WithBaseField) is not provided.

---

Also, this changes Gd::try_cast<U> from returning Option<Gd<U>> to Result<Gd<U>, Gd<T>>.

In other words, if the cast fails, Err(self) is returned by value. This allows you to re-use the original object for other cast attempts (e.g. when checking against multiple derived classes in a chain).

---

Other changes:

1. Callable::from_object_method() now takes its object parameter by shared-ref instead of value, no longer requiring the user to clone or submit ownership.
2. Docs and consistency/formatting changes.
3. Removes unused dependencies criterion and serde_json. The benchmark in godot-fmt can be manually re-enabled, but currently the dependencies slow down CI for no reason.
4. Adjusts sh check.sh clippy to run --all-targets like CI.
5. Removes remaining occurrences of bors. RIP 🪦

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-497

[lilizoey]

i think to_gd might be a better name, self.to_gd() feels more apt and rusty than self.self_gd(). not to mention it's a bit repetitive to do self.self_gd().

also in a situation like this:

fn process(&mut self, delta: f64) {
  self.some_reference.self_gd()
}

it seems a bit less obvious to me that self_gd refers to the self of some_reference and not to the self that's in scope.

[Bromeon]

I was also thinking about outer_gd or similar to express that we get the enclosing/wrapping pointer.
But we can go with to_gd for now, might want to re-evaluate this once there are upcoming Base changes.