Fix incorrect syn features (#44)

Author: dcchut

Cargo.toml

@@ -15,7 +15,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = { version = "1.0", default-features = false }
 quote = { version = "1.0", default-features = false }
-syn = { version = "2.0", features = ["full", "visit-mut"],  default-features = false }
+syn = { version = "2.0", features = ["full", "visit-mut", "parsing", "printing", "proc-macro", "clone-impls"],  default-features = false }
 
 [dev-dependencies]
 futures-executor = "0.3"