Do not use generic param defaults in impls

[rmartinho]

This will become an error in the future, and the defaults in impl are ignored anyway so it makes no sense to preserve them in the generated impl.

[kirillsemyonkin]

ngl first time looking into this derive code, it is kinda weird. could just rewrite to

#[proc_macro_derive(ImplicitClone)]
pub fn derive_implicit_clone(item: proc_macro::TokenStream) -> proc_macro::TokenStream {
    let mut input = syn::parse_macro_input!(item as syn::DeriveInput);
    input
        .generics
        .type_params_mut()
        .for_each(|type_param| type_param.bounds.push(syn::parse_quote! { ::implicit_clone::ImplicitClone }));

    let syn::DeriveInput {
        ident, generics, ..
    } = input;
    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
    let res = quote! {
        impl #impl_generics ::implicit_clone::ImplicitClone for #ident #ty_generics #where_clause {}
    };
    res.into()
}

this way we just add bounds, and rest syn will figure out on its own

[rmartinho]

I'm not super familiar with writing derive macros, so I tried to change as little as necessary.

I will try your implementation later and update the PR if there are no issues.