Objective-c categories aren't included in inheritance traits and impl blocks

Objective-c categories are a way of extending a given objective-c class. It almost feels like categories are used to organize various sections of their codebase. As a result, there are a lot of categories in the apple frameworks.

In #1750 I added better inheritance support for the binding generation but didn't add the impl blocks for the traits generated from the Objective-c categories.

It's unclear to me how to best implement this because a given class has no reference to the classes it implements. The only reference to the classes are from the categories to the classes they extend.

Input C/C++ Header

// bindgen-flags: --objc-extern-crate -- -x objective-c
// bindgen-osx-only

@interface Foo
-(void)method;
@end

@interface Foo (BarCategory)
-(void)categoryMethod;
@end

@interface Bar: Foo
@end

Bindgen Invocation

$ bindgen input.h --objc-extern-crate -- -x objective-c

Actual Results

/* automatically generated by rust-bindgen */

#![allow(
    dead_code,
    non_snake_case,
    non_camel_case_types,
    non_upper_case_globals
)]
#![cfg(target_os = "macos")]

#[macro_use]
extern crate objc;
#[allow(non_camel_case_types)]
pub type id = *mut objc::runtime::Object;
#[repr(transparent)]
#[derive(Clone, Copy)]
pub struct Foo(pub id);
impl std::ops::Deref for Foo {
    type Target = objc::runtime::Object;
    fn deref(&self) -> &Self::Target {
        unsafe { &*self.0 }
    }
}
unsafe impl objc::Message for Foo {}
impl Foo {
    pub fn alloc() -> Self {
        Self(unsafe { msg_send!(objc::class!(Foo), alloc) })
    }
}
impl IFoo for Foo {}
pub trait IFoo: Sized + std::ops::Deref {
    unsafe fn method(self)
    where
        <Self as std::ops::Deref>::Target: objc::Message + Sized,
    {
        msg_send!(self, method)
    }
}
impl Foo_BarCategory for Foo {}
pub trait Foo_BarCategory: Sized + std::ops::Deref {
    unsafe fn categoryMethod(self)
    where
        <Self as std::ops::Deref>::Target: objc::Message + Sized,
    {
        msg_send!(self, categoryMethod)
    }
}
#[repr(transparent)]
#[derive(Clone, Copy)]
pub struct Bar(pub id);
impl std::ops::Deref for Bar {
    type Target = objc::runtime::Object;
    fn deref(&self) -> &Self::Target {
        unsafe { &*self.0 }
    }
}
unsafe impl objc::Message for Bar {}
impl Bar {
    pub fn alloc() -> Self {
        Self(unsafe { msg_send!(objc::class!(Bar), alloc) })
    }
}
impl IFoo for Bar {}
impl IBar for Bar {}
pub trait IBar: Sized + std::ops::Deref {}

Expected Results

impl Foo_BarCategory for Bar {} is missing and should be something like the following:

...
impl IFoo for Bar {}
impl IBar for Bar {}
impl Foo_BarCategory for Bar {}
pub trait IBar: Sized + std::ops::Deref {}

Start with the Objective-C inheritance and category binding-generation paths described in the issue, using the supplied header and bindgen invocation as the reproduction case. Verify that a subclass receives the category trait implementation, with impl Foo_BarCategory for Bar {} appearing in the generated Rust output alongside the existing trait implementations.