Support cfg in declare_class!

[madsmtm]

Reported on winit matrix.

Code excerpt from that thread:

declare_class! {
    // -- snip --
    unsafe impl ApplicationDelegate {
        // -- snip --
        #[cfg(feature = "macos-open-urls")]
        #[sel(application:openURLs:)]
        fn open_urls(&self, _sender: *const Object, urls: &NSArray<NSURL, Shared>) {
            trace_scope!("application:openURLs:");
        }
    }
}

[madsmtm]

Implementation-wise: __attribute_helper's @extract_sel has to be extended to also extract attributes that apply to the function as well as any items referencing the function (which we do internally in code similar to classbuilder.add_method(ApplicationDelegate::open_urls).

Current list (based on the built-in index):

• cfg
• deprecated - We should add a corresponding allow(deprecated) before use
• Any availability attributes that we decide on: How do we handle availability? #266
• cfg_attr, if the inner attributes are on this list. (Might punt on this one, since it's a bit difficult, and a rare case anyhow)

[madsmtm]

We should similarly try to support such attributes on each impl item