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

Author: bkchr

reward-curve/Cargo.toml

@@ -15,7 +15,7 @@ targets = ["x86_64-unknown-linux-gnu"]
 proc-macro = true
 
 [dependencies]
-syn = { version = "1.0.7", features = ["full", "visit"] }
+syn = { version = "1.0.58", features = ["full", "visit"] }
 quote = "1.0.3"
 proc-macro2 = "1.0.6"
 proc-macro-crate = "0.1.4"