Add FromAttributes trait

[TedDriggs]

With trait declarations and impl blocks now supporting proc macros, there are too many items for darling to expose dedicated traits and derive macros for each one.

Requirements

• Easily specify macro ident/path for attribute filtering
• Merge fields across attribute declarations

Proposed API

pub trait FromAttributes: Sized {
    fn from_attributes(attrs: &[syn::Attribute]) -> darling::Result<Self>;
}

// in customer macro crate

#[derive(FromAttributes)]
#[darling(attributes(sample))]
pub struct SampleReceiver {
    // fields as usual
}

fn macro(input: TokenStream) -> TokenStream {
    let item_impl: syn::ItemImpl = syn::parse_macro_input!(input);
    let root_rec = SampleReceiver::from_attributes(&item_impl);
    // ... elided
}