DRILL-7092: Rename map to struct in schema definition
1. Renamed map to struct in schema parser.
2. Updated sqlTypeOf function to return STRUCT instead of MAP, drillTypeOf function will return MAP as before until internal renaming is done.
3. Add is_struct alias to already existing is_map function. Function should be revisited once Drill supports true maps.
4. Updated unit tests.
closes #1688
