Move AST::new() and AST::new_with_source() to internals.

src/serde/metadata.rs

@@ -272,7 +272,7 @@ impl Engine {
     }
 
     /// Generate a list of all functions in JSON format.
-    /// Available only under the `metadata` feature.
+    /// Exported under the `metadata` feature only.
     ///
     /// Functions from the following sources are included:
     /// 1) Functions registered into the global namespace