Add Option return types to Objective-c for nullability attributes

This is a subtask for #109 and I've been trying just implement it for a while but it's become clear that it's annoyingly nontrivial. Part of me writing this issue is so that it's at least documented.

Input Objc Header

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

@interface Bar
@end

@interface Foo
- (Bar* _Nullable)methodReturningBar;
- (Bar* _Nonnull)methodReturningNonnullBar;
@property (readonly, nonnull) Bar *nonnullBar;
@property (nullable, readonly) Bar *nullableBar;
@end

Bindgen Invocation

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

Actual Results

/* automatically generated by rust-bindgen 0.54.1 */

#[macro_use]
extern crate objc;
#[allow(non_camel_case_types)]
pub type id = *mut objc::runtime::Object;
#[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 IBar for Bar {}
pub trait IBar: Sized + std::ops::Deref {}
#[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 methodReturningBar(self) -> Bar
    where
        <Self as std::ops::Deref>::Target: objc::Message + Sized,
    {
        msg_send!(self, methodReturningBar)
    }
    unsafe fn methodReturningNonnullBar(self) -> Bar
    where
        <Self as std::ops::Deref>::Target: objc::Message + Sized,
    {
        msg_send!(self, methodReturningNonnullBar)
    }
    unsafe fn nonnullBar(self) -> Bar
    where
        <Self as std::ops::Deref>::Target: objc::Message + Sized,
    {
        msg_send!(self, nonnullBar)
    }
    unsafe fn nullableBar(self) -> Bar
    where
        <Self as std::ops::Deref>::Target: objc::Message + Sized,
    {
        msg_send!(self, nullableBar)
    }
}

The importart parts are the functions:

pub trait IFoo: Sized + std::ops::Deref {
    unsafe fn methodReturningBar(self) -> Bar
    where
        <Self as std::ops::Deref>::Target: objc::Message + Sized,
    {
        msg_send!(self, methodReturningBar)
    }
    unsafe fn methodReturningNonnullBar(self) -> Bar
    where
        <Self as std::ops::Deref>::Target: objc::Message + Sized,
    {
        msg_send!(self, methodReturningNonnullBar)
    }
    unsafe fn nonnullBar(self) -> Bar
    where
        <Self as std::ops::Deref>::Target: objc::Message + Sized,
    {
        msg_send!(self, nonnullBar)
    }
    unsafe fn nullableBar(self) -> Bar
    where
        <Self as std::ops::Deref>::Target: objc::Message + Sized,
    {
        msg_send!(self, nullableBar)
    }
}

Expected Results

Ideally, this should return Options around things that can be null

pub trait IFoo: Sized + std::ops::Deref {
    unsafe fn methodReturningBar(self) -> Option<Bar>
    where
        <Self as std::ops::Deref>::Target: objc::Message + Sized,
    {
        msg_send!(self, methodReturningBar)
    }
    unsafe fn methodReturningNonnullBar(self) -> Bar
    where
        <Self as std::ops::Deref>::Target: objc::Message + Sized,
    {
        msg_send!(self, methodReturningNonnullBar)
    }
    unsafe fn nonnullBar(self) -> Bar
    where
        <Self as std::ops::Deref>::Target: objc::Message + Sized,
    {
        msg_send!(self, nonnullBar)
    }
    unsafe fn nullableBar(self) -> Option<Bar>
    where
        <Self as std::ops::Deref>::Target: objc::Message + Sized,
    {
        msg_send!(self, nullableBar)
    }
}

This should also apply to parameters in functions. I've kept the nonnull functions because that's a very important use case that we need to preserve.

Start by reproducing the issue with the supplied input.h and bindgen invocation, then trace how Objective-C method and property nullability reaches the generated Rust signatures. Done means nullable object returns and parameters use Option while nonnull declarations remain unchanged, including the shown methods and properties.