DRILL-1171: Create Drill's implementation of ReduceAggregatesRule.
DRILL-1342: Fix nullability handling of aggregate functions.

Implement CastHigh function

Simplify CastHigh

Remove extraneous @Ignore in test (seems to be introduced by a rebase/merge).

Remove SumHighPrecision since it is not used by DrillReduceAggregates anymore.

Revert nullabie output types for decimal aggrs.