Revert "Replace some manual parsing code with darling helper crate"

This reverts commit e5af4f8efe.

The `darling` crate limits us to `syn::Meta` which is probably not
enough in the long run. There are a couple of other problems I have
with this crate and I think it's currently not worth. I'm considering
writing my own alternative...

macro/src/lib.rs

@@ -3,19 +3,14 @@ use proc_macro::TokenStream as TokenStream1;
 
 mod gen;
 mod ir;
-mod parse;
 mod util;
 
 
 #[proc_macro_derive(Config, attributes(config))]
 pub fn config(input: TokenStream1) -> TokenStream1 {
-    let input = match syn::parse2::<syn::DeriveInput>(input.into()) {
-        Err(e) => return e.to_compile_error().into(),
-        Ok(i) => i,
-    };
-
-    ir::Input::from_ast(input)
+    syn::parse2::<syn::DeriveInput>(input.into())
+        .and_then(ir::Input::from_ast)
         .map(gen::gen)
-        .unwrap_or_else(|e| e.write_errors())
+        .unwrap_or_else(|e| e.to_compile_error())
         .into()
 }