Add attribute for Equatable protocol

[NiwakaDev]

This PR introduces the #[swift_bridge(Equatable)] attribute, which is used to generate a type that conforms to Equatable protocol.

Somthing like:

// Rust 
mod ffi {
    extern "Rust" {
        #[swift_bridge(Equatable)]
        type SomeType;
    }
}

// Generated Swift
class SomeType {
    // ...
}

// SomeType inherits SomeTypeRef. So, SomeType also conforms to Equatable protocol.
extension SomeTypeRef: Equatable {
    public static func == (lhs: SomeTypeRef, rhs: SomeTypeRef) -> Bool {
        __swift_bridge__$SomeTypeType$_partial_eq(rhs.ptr, lhs.ptr)
    }
}

[chinedufn]

Looks perfect. Tests look great. Thanks so much for your work here. Great stuff.

---

Can you just update your PR body with an example of the feature.

Here's an example: #21

You can see more examples in the releases page https://github.com/chinedufn/swift-bridge/releases

Then I can merge this.

[chinedufn]

Oh, and we need to add this to the attributes documentation here

swift-bridge/book/src/bridge-module/opaque-types/README.md

Lines 136 to 170 in 607e305

	## Opaque Type Attributes
	#### #[swift_bridge(already_declared)]
	The `already_declared` attribute allows you to use the same type in multiple bridge modules.
	```rust
	use some_crate::App;
	mod ffi {
	extern "Rust" {
	type App;
	#[swift_bridge(init)]
	fn new() -> App;
	}
	}
	#[swift_bridge::bridge]
	#[cfg(feature = "dev-utils")]
	mod ffi_dev_utils {
	extern "Rust" {
	// We won't emit Swift and C type definitions for this type
	// since we've already declared it elsewhere.
	#[swift_bridge(already_declared)]
	type App;
	fn create_logged_in_user(&mut self, user_id: u8);
	}
	}
	```
	#### #[swift_bridge(Copy($SIZE))]
	If you have an opaque Rust type that implements `Copy`, you will typically want to be

You can put Equatable below the Copy attribute

[NiwakaDev]

Can you just update your PR body with an example of the feature.

I did this. Please check my PR body!

[NiwakaDev]

Oh, and we need to add this to the attributes documentation here

Sure. I'll address this tomorrow!

[NiwakaDev]

Hi, @chinedufn!

I addressed your review.

[chinedufn]

Thanks!

[chinedufn]

Referenced this PR in the guide for #269

Thanks!