Add adapter newtype or macro option to work with syn::Parse

[TedDriggs]

The syn::parse_macro_input! macro and synattra crate both use the syn::Parse trait to do their work. Anything that implements one of darling's traits should be able to implement that. A blanket impl is probably impossible, so the choices are:

1. Provide a generic newtype wrapper and implement it on that.
2. Add a new option to the code generator to also implement the Parse trait.
3. Do both.

[TedDriggs]

This doesn't seem to be as feasible as I'd hoped, so in the absence of demand I'm closing this.