Convert ptr, count arguments into slice pointers

[madsmtm]

Convert things like:

#[method_id(initWithObjects:count:)]
pub unsafe fn initWithObjects_count(
    this: Allocated<Self>,
    objects: *mut NonNull<ObjectType>,
    cnt: NSUInteger,
) -> Retained<Self>;

#[method_id(colorWithColorSpace:components:count:)]
pub unsafe fn colorWithColorSpace_components_count(
    space: &NSColorSpace,
    components: NonNull<CGFloat>,
    number_of_components: NSInteger,
) -> Retained<NSColor>;

#[method_id(managedObjectIDFromUTF8String:length:)]
pub unsafe fn managedObjectIDFromUTF8String_length(
    &self,
    utf8string: NonNull<c_char>,
    len: NSUInteger,
) -> Retained<NSManagedObjectID>;

Into:

pub unsafe fn initWithObjects(
    this: Allocated<Self>,
    objects: *mut [NonNull<ObjectType>],
) -> Retained<Self> {
    let cnt = objects.len();
    let objects = objects.as_mut_ptr();
    unsafe { msg_send_id![this, initWithObjects: objects, count: cnt] }
}

pub unsafe fn colorWithColorSpace_components(
    space: &NSColorSpace,
    components: NonNull<[CGFloat]>,
) -> Retained<NSColor> {
    let number_of_components = components.len();
    let components = components.as_non_null_ptr();
    unsafe { msg_send_id![NSColor::class(), colorWithColorSpace: objects, components: components, count: number_of_components] }
}

pub unsafe fn managedObjectIDFromUTF8String(
    &self,
    utf8string: NonNull<[c_char]>,
) -> Retained<NSManagedObjectID> {
    let len = utf8string.len();
    let utf8string = utf8string.as_non_null_ptr();
    unsafe { msg_send_id![self, managedObjectIDFromUTF8String: utf8string, length: len] }
}

That is, methods that have a pointer argument and uses count: with a NSInteger/NSUInteger should instead allow passing a slice pointer, and then we extract the count automatically.

This probably requires that we output a shim for the method manually, as doing this kind of rewriting purely in the type-system is very difficult.

Unsure how this should work in declare_class!? Maybe we do need to integrate it into the type-system? Maybe we need a special attribute that says where the count should go?