Rename parse_tablename -> parse_object_name (4.2/4.4)

...to match the name of the recently introduced `SQLObjectName` struct
and to avoid any reservations about using it with multi-part names of
objects other than tables (as in the `type_name` case).

tests/sqlparser_postgres.rs

@@ -99,14 +99,14 @@ fn parse_complex_insert() {
 #[test]
 fn parse_invalid_table_name() {
     let mut parser = parser("db.public..customer");
-    let ast = parser.parse_tablename();
+    let ast = parser.parse_object_name();
     assert!(ast.is_err());
 }
 
 #[test]
 fn parse_no_table_name() {
     let mut parser = parser("");
-    let ast = parser.parse_tablename();
+    let ast = parser.parse_object_name();
     assert!(ast.is_err());
 }