Fix incorrect use of syn::exports (#7838)

* Fix incorrect use of syn::exports

Instead of using `syn::exports` we should import the trait from the
quote crate directly.

* Use own macro for test cases to fix compilation with latest syn

* Fix test

substrate/frame/support/procedural/src/storage/parse.rs

@@ -47,7 +47,7 @@ mod keyword {
 pub struct Opt<P> {
 	pub inner: Option<P>,
 }
-impl<P: syn::export::ToTokens> syn::export::ToTokens for Opt<P> {
+impl<P: quote::ToTokens> quote::ToTokens for Opt<P> {
 	fn to_tokens(&self, tokens: &mut proc_macro2::TokenStream) {
 		if let Some(ref p) = self.inner {
 			p.to_tokens(tokens);