swarm-derive/: Derive Debug for generated OutEvent (#2821)

When generating an `OutEvent` `enum` definition for a user, derive `Debug`
for that `enum`.

Why not derive `Clone`, `PartialEq` and `Eq` for the generated `enum`
definition?

While it is fine to require all sub-`OutEvent`s to implement
`Debug`, the same does not apply to traits like `Clone`. I
suggest users that need `Clone` to define their own `OutEvent`.

swarm-derive/src/lib.rs

@@ -159,6 +159,7 @@ fn build_struct(ast: &DeriveInput, data_struct: &DataStruct) -> TokenStream {
                     let visibility = &ast.vis;
 
                     Some(quote! {
+                        #[derive(::std::fmt::Debug)]
                         #visibility enum #name #impl_generics
                             #where_clause
                         {