Remove dependency on syn/visit feature

The builtin visitor is fairly expensive to compile (3700 lines of code),
particularly if something else in the dependency graph also enables
syn/full. For the usage in serde_derive, it turns out to be easy to
replace.

serde_derive/Cargo.toml

@@ -22,7 +22,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = "1.0"
 quote = "1.0"
-syn = { version = "1.0.58", features = ["visit"] }
+syn = "1.0.60"
 
 [dev-dependencies]
 serde = { version = "1.0", path = "../serde" }