Initial impl of union coercion

Author: friendlymatthew

datafusion/core/tests/sql/mod.rs

@@ -71,6 +71,8 @@ mod runtime_config;
 pub mod select;
 mod sql_api;
 
+mod union_comparison;
+
 async fn register_aggregate_csv_by_sql(ctx: &SessionContext) {
     let testdata = test_util::arrow_test_data();