DRILL-1062: Implemented null ordering (NULLS FIRST/NULLS LAST).
Primary:
- Split "compare_to" function templates (for sorting) into
"compare_to_nulls_high" and "compare_to_nulls_low" versions.
- Added tests to verify ORDER BY ordering.
- Added tests to verify merge join order correctness.
- Implemented java.sql.DatabaseMetaData.nullsAreSortedHigh(), etc.
Secondary:
- Eliminated DateInterfaceFunctions.java template (merged into other).
- Renamed comparison-related template data objects and file names.
- Eliminated unused template macros, function template classes.
- Overhauled Order.Ordering; added unit test.
- Regularized some generated-class names.
Miscellaneous:
- Added toString() to ExpressionPosition, Order.Ordering, JoinStatus.
- Fixed some typos.
- Fixed some comment syntax.
