Minor refactors to make wiggle-generate reusable externally (#34)

* wiggle-generate: pass witx doc in explicitly

* wiggle-generate: Names takes &Config, minor refactor for reuse

src/lib.rs

@@ -6,5 +6,6 @@ use syn::parse_macro_input;
 #[proc_macro]
 pub fn from_witx(args: TokenStream) -> TokenStream {
     let config = parse_macro_input!(args as wiggle_generate::Config);
-    TokenStream::from(wiggle_generate::generate(config))
+    let doc = witx::load(&config.witx.paths).expect("loading witx");
+    TokenStream::from(wiggle_generate::generate(&doc, &config))
 }